Questions 121 to 128
This section talks about what the Church is and the role of the Church and its members.
See What does Church mean?
Go to Lumen GentIum (from Vatican II on the Church):
He [God] planned to assemble in the holy Church all those who would believe in Christ. Already from the beginning of the world the foreshadowing of the Church took place. It was prepared in a remarkable way throughout the history of the people of Israel and by means of the Old Covenant.(1*) In the present era of time the Church was constituted and, by the outpouring of the Spirit, was made manifest. At the end of time it will gloriously achieve completion, when, as is read in the Fathers, all the just, from Adam and “from Abel, the just one, to the last of the elect,”(2*) will be gathered together with the Father in the universal Church.
The essential nature and task of the Church:
… [T]he only truly independent mind … [is] the Church. And that keeping her company, entering into her, entrusting yourself to her faith–which is allegedly being nothing but infantile and dependent–represents in reality the greatest degree of independence from the spirit of the age and signifies greater boldness than is embodied in any other possible position. …That is not an infantile dependence; that is courage to contradict and the freedom to go against prevailing opinions, the freedom that offers us a firm footing and which the Church has not invented for herself.’ – Cardinal Ratzinger (Full text)
To gain a better understanding of this concept, read the comments section of this blog called Institution of the Catholic Church by Christ.
See Vatican II: the Church as the people of God.
Read the Encyclical on the Mystical Body of Christ:
13. If we would define and describe this true Church of Jesus Christ – which is the One, Holy, Catholic, Apostolic and Roman Church  – we shall find nothing more noble, more sublime, or more divine than the expression “the Mystical Body of Christ” – an expression which springs from and is, as it were, the fair flowering of the repeated teaching of the Sacred Scriptures and the Holy Fathers.
On the Temple of the Holy Spirit.
• How does faith enlighten reason?
See Augustine: Reason and Faith, Philosophy and God
• What is transhumanism?
See Transhumanists Selling their Technological “Utopia” to Christians