You are asking about a very urgent and important practice, in other words, a protection for developing one’s mind on the path to enlightenment.

It is said in many tantric teachings—such as the Kalachakra and Guhyasamaja—that even if one has accumulated the five uninterrupted negative karmas, one can still achieve the sublime vehicle in this life, in particular the Maha-anuttara path, which is the most skillful for granting enlightenment in a brief lifetime in degenerate times. But those who criticize the guru from the heart will not achieve this, even if they practice the sublime vehicle.

In Lama Tsongkhapa’s lamrim, it is clearly mentioned that even for the thought that the virtuous friend is ordinary to arise is a cause to lose realizations. This means also that it becomes an obstacle to developing the mind on the path.

The very important thing is to analyze and check as much as possible before making Dharma contact. With the recognition of guru and disciple, since the Dharma contact is established, then there is no going back. One has to have a new relationship. It is another world, looking at that person with a new mind, a pure mind.

It is said by Pabongka Dechen Nyingpo, the great enlightened being, the Heruka, that if one is able to stop all thoughts of mistakes and look only at good qualities, seeing the guru only as Buddha, then one can achieve enlightenment in this life.

This is not only for this life. Generally, one cannot get enlightened while seeing mistakes in the virtuous friend. But, with the realization of seeing all buddhas as the guru and all gurus as Buddha, one can. This is said in all four Tibetan Mahayana sects, in both sutra and tantra.

Making mistakes, allowing heresy, anger, or criticism to arise, giving up the virtuous friend: these become the cause for one not to find a guru in future lives. It is said in the Essence of Nectar that it causes one to be unable to hear the sound of the holy Dharma, not to mention being able to find a virtuous friend, and that one becomes impoverished in terms of a virtuous friend in all one’s lifetimes.

This is the last outline of the eight shortcomings of incorrectly devoting to a virtuous friend.

I often mention this quotation by the Fifth Dalai Lama:

When one’s own mistakes appear to one’s own hallucinated mind in the actions of the guru, one must realize that this is one’s own mistake and abandon it like poison.

This means that we must abandon the belief that there is a mistake in the actions of the virtuous friend.

With this mindfulness, look at that person as Buddha, one who has eliminated all mistakes and has all good qualities. Then, if the guru asks you to do something, and it’s something that you don’t have the capacity to do now, if your mind hasn’t reached that level, then with this pure thought that I mentioned before, with this mindfulness, one respectfully explains to the guru that one is incapable of doing this, and in this way one tries to get the guru to excuse one from doing it.

This is what is mentioned in the Fifty Verses of Guru Devotion and in the Vinaya. If the guru says to do something that is not Dharma, one can ask permission not to do it. It doesn’t say to have negative thoughts or to criticize him. This is how to deal with the problem without it becoming an obstacle to developing one’s mind on the path.

Of course, as His Holiness the Dalai Lama mentions all the time, when a special guru and special disciple meet, then every single thing the guru says is to be done, like Tilopa and Naropa, or Marpa and Milarepa, and so forth.

The main thing here is to be skillful and try not to hurt the holy mind of the guru. Hurting the mind of the guru is the greatest obstacle to developing one’s own mind on the path to enlightenment.

If you have made a mistake toward the guru then you must confess from the heart. This way, you get the profit of lightening and purifying the negative karma. Of course, there are many other practices, such as Samayavajra and self-initiations, but one very good thing is to do some service, some practice that pleases the holy mind of the virtuous friend.

You mentioned sincerity, but in my view this sincerity is without wisdom. Sincerity is beneficial to others when it has wisdom and is performed with compassion. If there is wisdom, usually there should also be compassion, I think, which means a very wide, encompassing wisdom.

For example, if someone comes along who wants to kill a particular person, and he asks you where that person has gone, and you think you should be sincere, without wisdom—which also should include compassion—then you will tell him where the person went, instead of saying you don’t know, which would save the person’s life.

The other thing is, we can always find mistakes. Also, we think it is a question of whether it is a big mistake or a small mistake. There will always be something —too much anger, too much miserliness, too much pride, a partial mind, etc.

If you look for mistakes, you will see mistakes.