Father Clérissac was the trusted spiritual director of Jacques and Raïssa Maritain. At some point, he got into Action Française, a right-wing political movement, and attracted the Maritains toward it. They eventually moved away from Action Française, repudiated it.
Because Fr. Clérissac was trustworthy overall, he was able to introduce currents into his friends' lives that they came to regret later on. So there is a warning in this for anyone who seeks to become more trustworthy, that they might be creating the conditions to mislead other people.
Also there is a warning in this for anyone who considers themselves trustworthy and wishes to avoid being ambitious in becoming trustworthy. Whether you want to be trustworthy or not, you are, to some people and in some ways. So the untrustworthy aspects of yourself that you don't do anything about may betray those people.