There are three types of teachers:
1. One who knows the shAstra-s and is also firmly established in that knowledge, i.e. a shrotriya and a brahma niShThA.
2. One who does not know the shAstra-s but is firmly established in the knowledge, i.e. only a brahma niShThA.
3. One who knows the shAstra-s but is not firmly established in the knowledge, i.e. only a shrotriya.
Of these, the first one is the best and the scriptures advise one to approach such a teacher.
The second is a problem since, although he may be a brahma niShThA, he is not a teacher. Shankara says that one should avoid such a teacher. He may inspire one to inquire but he cannot guide others.
The third one is the second best, since at least one can learn from that teacher.
Now the problem is that a brahma niShThA alone will know that he is a brahma niShThA – no one else can know. One can only guess by studying the persons behavior for a prolonged length of time; observing that he does not get excited, for example, as Krishna describes.
This means that the student can never know if his teacher is a brahma niShThA or not but he can find out if his teacher has studied the scriptures or not.
Since one needs shraddhA or faith in the words of the teacher, it is imperative for the student to assume that his teacher is realized (irrespective of whether the teacher is realized or not). Then, if he learns, the student can realize since he has faith in the teaching.
Ultimately, a teacher is one who directs the disciple to the scriptures and not to himself as the authority, since the scriptures ultimately form the means of knowledge.
Now the question is: why do we need a teacher or guide? Why cannot just a book or tape recorder or CD player be a teacher?
Firstly, this is a subjective science involving non-objectifiable truths. The preconceived notions of the seeker form impediments to knowledge. If we cannot do a PhD without a guide, as stipulated by any university rules, it is even more imperative to have a guide for this more subtle inquiry. Furthermore, simply having a PhD does not mean that one can guide another student. Either one has to have both a PhD and be involved in continuing inquiry or research or one may be well established in research but not actually have a PhD. The same thing applies to this more subtle inquiry.
Hence the Vedic instruction is that we should always approach a teacher for the knowledge.
Ultimately, it is due to the merits of many lives that one can find a proper teacher who can guide us to salvation.
Hence, the vivekachUDAmaNi says:
manushyatvam mumukShutvam mahApurushasamsrayam – dulabham – daiva anugraham
Being born as human being with discriminative faculty,
having a desire to become free from bondage and
having the acquaintance of a great soul.
All these three are rare indeed and it is only due to the grace of God that it can happen.
A student has to discover his teacher for himself; no teacher will come and say I am your teacher. A student will discover his teacher when he has full faith in the teaching. Without that full faith, no teaching can takes place. ShraddhAvan labhate j~nAnam says Krishna.