In a wide perspective, a religion is the belief for a perfect conduct, which leads to a karmic free life. In other words, it's a proven path to attain salvation when its principles are strictly followed.
While following a religion, it becomes very essential and also demands real wisdom to understand and accept a guru. Scriptures and Scholars say, when the real search starts within, and when the right time hits, then the Guru will automatically approach the person looking for guidance. It's undoubtedly true. At the same time, when there are situations to choose a guru on our choice, then it's a question involving lots of thinking the pros and cons before accepting the right one.
In countries like India, where Religion is the base of life for people, there are ample chances for crooks to play with ignorance of people. This is a country where even cine actors or actress are being regarded equivalent to God. Hero-worship is one of the phobias that misguides people to disaster. People who are even well educated, and have a good thinking succumb to such crooks. Not all come under this category but from time to time some have been brought to light.
A sanyasin or sanyasi is one who is said to have renounced everything. There are certain principles for a sanyasi or a renounced, such as, he should get alms and eat, he should not use any comforts to sleep, he should travel only by foot, and so on. Now a days, how many sanyasins are doing so? Very importantly, Gurus like Adhi Sankarachariar and Sri Ramanujar have a lineage [dealt in detail below]. People who accept a guru should invariably see his lineage or parampara or the mutt the guru belongs to. Never ever accept anyone as guru if he or she doesn't have any guru, and does not come under any lineage. Only a few initiated people are recognized by the hierarchy to initiate, and to become a guru. Most of the initiated devotees are not allowed to initiate others. So, take due care before approaching or accepting one as guru.
Scriptures and Bagavatham say that Lord Krishna had appeared in the human form in this world. To the Lord himself, Sandipani was his Guru. Though he had proved innumerable times that he's no more an ordinary soul, there was a guru for him. If this applies to Lord Krishna himself, then it clearly applies to everyone else who wishes to guide others.