This topic is obviously well discussed, and it?s a controversial one. Here’s the most often-cited reference (from Srimad Bhagavatam 8.21. text 1 purport): “Anyone who is supposed to be a guru but who goes against the principle of visnu-bhakti cannot be accepted as a guru. If one has falsely accepted such a guru, one should reject him. Such a guru is described as follows (Mahabharata, Udyoga 179.25) guror apy avaliptasya, etc.). Srila Jiva Gosvami has advised that such a useless guru, a family priest acting as guru, should be given up, and that the proper, bona fide guru should be accepted sat-karma-nipuna vipro, etc.”