The symbols of the Tetramorph, the Lion, Bull, Eagle and Angel, the collective four symbols originating from Greek astrology which are associated with the New Testament gospel authors, sometimes with Christ at their centre, are an ancient, potent and complex, religious symbol.
Depictions of the tetramorph (from Greek tetra four, morph shape) can be found in Christian art such as illuminated manuscripts, engravings and stained glass in churches from the Middle Ages to the present-day. The significance of their being four in number occurs quite early in Christianity. The early church Father and bishop of Lyons, Saint Irenaeus (end of 2nd century CE - c. 202 CE ) declaring -
'It is not possible that the Gospels can be either more or fewer in number than they are. For, since there are four zones of the world in which we live, and four principal winds, while the… “pillar and ground” of the Church is the Gospel and the spirit of life; it is fitting that she should have four pillars, breathing out immortality on every side…. He who was manifested to men, has given us the Gospel under four aspects, but bound together by one Spirit.'
However, it was Saint Jerome (370 -420 CE) who's credited as the first to designate the Greek zodiac symbols of the Bull, Eagle, Lion and Angel, as emblematic of the four Gospel authors. Jerome’s designated the three animals and one human form with their associated virtues as exemplary of specific attributes of Christ.
In modern times, the Swiss psychologist C.G. Jung noted of the Tetramorph in which Christ is sometimes depicted at its centre,
'He (Christ) holds an important position midway between the two extremes, man and God, which are so difficult to unite. ..He is lacking in neither humanity nor in divinity, and for this reason he was long ago characterized by totality symbols, because he was understood to be all-embracing and to unite all opposites. The quaternity of the Son of Man, indicating a more differentiated consciousness, was also ascribed to him (via Cross and tetramorph)'. 
C.G. Jung recognised that a four-fold pattern dating from prehistory was of near universal occurrence in world art and religion. In ancient Egyptian mythology the god Horus is accompanied by his four sons, while the three animals and human form of the tetramorph, first mentioned by the prophet Ezekiel's vision (Ez.1:10) is now recognised as alluding to the Sumerian zodiac. The universal occurrence of a four-fold design representing a totality, is commented upon by C.G.Jung thus-
'The quaternity is an organizing schema par excellence, something like the crossed threads in a telescope. It is a system of coordinates that is used almost instinctively for dividing up the visible surface of the earth, the course of the year, or the collection of individuals into groups, the phases of the moon, the temperaments, elements, alchemical colours, and so on.' Elsewhere Jung states, 'the four quarters of heaven, the four elements are a quaternary system of orientation which always expresses a totality...the orientating system of consciousness has four aspects, which correspond to four empirical functions: thinking, feeling, sensation (sense-perception), intuition. This quaternity is an archetypal arrangement...' 
Remarkably, a quaternity comprising of four quite distinct entities, namely body and mind, spirit and soul, can be found in Christ’s commandment in the Gospels of Luke and Mark. 
'And thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength; this is the first commandment'.
In Christ's commandment, adapted from the Jewish Shema, two of the four entities in the totality of human life, mind and soul are named. Of the heart it’s worth considering the crucial role which the anatomical organ has symbolically to kingship, the lion and the Spirit. One thinks of the Crusader King Richard the Lionheart for example, while in astrological symbolism the zodiac sign of Leo rules the heart and is associated with fire. In Judaic and Christian symbolism fire is frequently associated with encounters with the Divine and with the Holy Spirit. In the non canonical gospel of Thomas (340 CE) has Christ declare, 'He who is near unto me is close to the Fire', while in the Gospel of Luke, Christ says, 'I am come to send fire on the earth; and what will I, if it be already kindled?'
Religious symbolism involving fire in the final analysis originates from the fire-worshipping Zoroastrian religion of Persia. The qualities of Courage, kindness and love in its various guises, are also associated with the heart, all of which are also related to Spirit.
Strength can confidently be identified as comparative to the physical world. The exemplary animal associated with strength, the Bull or Ox, has a legendary enduring strength which serves it to even commit an act of self-sacrifice. Strength is predominantly associated with muscular activity and the physical realm, and above all in element has an earthy quality to it.
The mental faculty of thought, along with the imagination is associated with the element of Air in various symbolic schemata. Ideas are sometimes described as being plucked out of air, while the phrase to have one's head in the clouds also suggests a relationship between the mind and Air.
The Soul is often described as passive, receptive, feminine and as 'The Other', usually by male theological commentators. Dissolution and hidden depths are also related to both the soul and the element of Water. Thus a quaternity involving a totality of body and mind, spirit and soul occurs within Christ’s commandment.
There's also the extraordinary idea that the 'clock number' of the four 'fixed' zodiac signs Taurus (2), Leo (5), Scorpio (8) and Aquarius (11) when added up total 26, the very same number in Hebrew Gematria for the Tetragrammaton JHVH in which Yod (10), Hey (5), Vav (6) and Hey (5) also equal 26.
In a tetramorph dating from 1482 (picture below) Christ is depicted as the ruler of the four elements.
Ultimately however, the symbolism of the tetramorph originates from the Babylonian zodiac, specifically the so-called 'Fixed Cross’ of astrological signs in opposition and right-angles to each other, Taurus representing Earth and its associations, Leo and the element of Fire, Scorpio for Water  and Aquarius as representative of Air.
The Christian tetramorph is a superb example of syncretism, that is, how religions and beliefs sometimes overlap each other, and how old symbols are adopted for newer beliefs, sometimes quite different from their origins.
One of the 20th century's greatest scholars of religious symbolism, C.G. Jung, held a great regard for his fellow compatriot, the Renaissance alchemist-physician Paracelsus. Besides being an early pioneering advocate for the use of chemical remedies in medicine and a theologian as radical and original as his contemporary, Martin Luther, Paracelsus (1493-1541) was also a proto-psychologist. In an essay entitled Paracelsus as a Spiritual Phenomenon (1942) C.G. Jung delved into the questing and confused world of Paracelsus’s four mysterious Scaiolae. Jung first consults the Dictionary of Alchemy (1612) by Martin Ruland (1569-1611), a Paracelsian scholar and lexicographer who was resident at the Prague court of the alchemy-loving Holy Roman Emperor, Rudolph II (1552 - 1612).
Ruland defines the Paracelsian Scaiolae as - 'Spiritual Powers of the Mind, its properties and virtues, which are fourfold, according to the number of the elements, and the four wheels of fire which were part of the Chariot in which Elias was taken up to Heaven. They emanate from the soul in man. Fancy, imagination, speculative faculty, etc., are included under the term. It also embraces, in a special sense, the Articles of our Christian Faith in Jesus Christ, Baptism, partaking of the Eucharist, Charity towards our neighbour, manifesting the perfect Fruits of Faith, whereby we attain not merely prolonged but eternal life.' 
Jung continues, 'Ruland interprets the four first psychologically, as phantasia, imaginato, speculatio, and agnata fides (inborn faith).. Since every archetype is psychologically a fascinosum, i.e., exerts an influence that excites and grips the imagination, it is liable to clothe itself in religious ideas.... it would not be overbold to conclude that the four Scaiolae correspond to the traditional quadripartite man and his all-encompassing wholeness. The quadripartite nature of the homo maximus is the basis and cause of all division into four: four elements, seasons, directions etc... [ibid]
Jung also consulted the writings of Gerard Dorn (1530-84), a Belgian philosophical alchemist, who like Ruland, was an advocate of Paracelsian ideas. Dorn emphasized the psychic nature of the Scaiolae (“mental powers and virtues, properties of the arts of the mind”)...these external principles, of the invisibilis homo maximus. The four Scaiolae appear to be interpreted by Dorn as mental powers and psychological functions. 
Finally, connecting the function of the four Christian Gospels to the proto-psychology of Paracelsus, Jung declares - 'The Scaiolae, as the four parts, limbs or emanations of the Anthropos are the organs with which he actively intervenes in the world of appearances or by which he is connected with it, just as the invisible quinta essentia, or aether, appears in this world as the four elements or conversely, is composed out of them. Since the Scaiolae, as we have seen, are also psychic functions, these must be understood as manifestations or effluences of the One, the invisible Anthropos. As functions of consciousness, and particularly as imaginato, speculatio, phantasia, and fides, they “intervene”. 
C.G. Jung devoted the last thirty years of his life to the study of alchemy and its symbolism. His belief in man as essentially a religious animal who quests for meaning and purpose in their own unique, individual life, has lost no relevance today. Jung's profound study of comparative religion, in conjunction with his consultation and analysis of his many patients, along with the events of two World Wars, led him to the conclusion that all too few experience the living Christ within their own lives, a lack often hindered by fossilized Christian dogma.
Revealingly, in regard to the four-fold design of the quaternity and the central figure of the Tetramorph, Jung concluded that - 'The Gnostic quadripartite original man as well as Christ Pantokrator is an imago lapidis.  Its also enlightening to visit the church of Saint John the Baptist's, Maddermarket, Norwich, with Jung's thoughts upon the quaternity as an image of the Philosopher's stone, held in mind. Sir Thomas Browne's exhortation at the apotheosis of his hermetic phantasmagoria, the discourse The Garden of Cyrus (1658) in which the physician-philosopher encourages his reader, 'to search out the Quaternio's and figured draughts of this order' also seems apt.
No less than three examples of the tetramorph can be viewed within the church; in its East window, accompanied by respective Evangelist, high up in the bell-tower of its West window, and also carved upon its Nave processional gates. The church houses a fourth example of a quaternity, a highly original and sophisticated variant upon the theme of the quaternity of the homo maximus. Encased within the two pilasters of the early seventeenth century marble funerary monument the Layer monument are four figurines exemplary of Paracelsian scaiolae. The upper pair represent the two eternal rewards for the Christian, Pax and Gloria (Peace and Glory). Its lower pair of figurines represent mortal psychic entities, one of which is positive and one of which is negative, Vanitas and Labor (Vanity and Labour). At the centre of the monument there is a large skull.
C.G. Jung identified Christ as none other than a symbol of the self. Another symbol which predates the Christian era, but which is equally potent as a symbol of the Self, is the skull. Besides being universally recognised as a momento mori symbol, the skull is also associated in alchemy with the Vas Philosophorum, the philosophical vessel and the place where the opposites reside, clash and are reconciled. The skull in alchemical symbolism is also where the incubation of the Philosopher’s Stone occurs and where the homo maximus or greater man within, more often than not either slumbering or invisible, dwells.
In the final analysis discussion upon the quaternity of the Tetramorph can never be or exhausted or its significance in religious and psychological terms explained; for like all living symbols, it will always transcend interpretative attempts. However, the original Greek definition of a symbolon as a tally-stick, coin or object broken into two halves used for identification, recognition or completeness when united, greatly assists our understanding; for Man only ever holds one half of the broken coin, tally stick, or object, the other, 'invisible’ or missing half' of the symbolon, is firmly held by God.
 Collected Works vol 10 paragraph 692
 CW 9 ii paragraph 381
 Luke 10 v. 27 and Mark 12 v. 30
 Just how and why the astrological sign of Scorpio, the 'King' of the Insects is replaced by the regal and heaven-inhabiting King of the birds, the Eagle, goes beyond the confines of this short essay !
 Martin Ruland Lexicon Alchemia (1612) is in Sir Thomas Browne's library p. 22 no. 119
as is Paracelsus Opera (1603) p. 22 no.118 as well as Gerard Dorn in Theatrum Chemicum
(5 vols. 1613) page 25 no. 124
 'Paracelsus as a Spiritual Phenomenon' (1942)
subsection C. The Quaternity of the Homo Maximus CW 13: paragraphs 206-208
 CW vol. 13 paragraph 215
 CW vol. 12 paragraph 173
Top - Medieval illuminated manuscript example from Bode Museum Germany
Next - Glanville - Le Proprietaire des choses (1482)
Next - Leonhard Thurneysser (1531-96) the Hermaphrodite from Quinta Essentia 1574
Bottom - Realization of the Layer monument as a Quaternity and with skull as a Quincunx, Norwich, circa 1600.
Collected works of Carl Gustav Jung - volumes 9 i, 12 and 13. pub. RKP
Catalogue of the libraries of Sir Thomas Browne and his son Edward. pub. E.J. Brill 1986
Faulkner, Kevin - The Layer Monument- An Introduction and Interpretation as an Alchemical Mandala. Pride Press 2013