My opinion of psychology is that properly the term refers to the descriptive study of human behavior. So in and of itself, it could not be illegitimate, in fact every one of us is engaging in psychology when we impute motivations to another person. However, the state of modern psychology is obviously ungodly and illegitimate, not so much because it is psychology, but because it is thoroughly secular. If you read modern psychology papers they are absolutely marinated in the secular worldview and it is the influence of their secular presuppositions which causes their obscene beliefs and behaviors eg sanctifying drag queen story hour and other crimes. Normally you get the impression they are unaware any alternative even exists and take their presuppositions completely for granted, revealing a lack of self-awareness and leading to a tendency to arrogantly think they are merely correct and no rational argument could exist (ironically, it is the secularists who blindly follow established dogma). They also have a tendency to arrogantly exclude themselves from their own theories, reinforcing their misplaced faith in their own ideas and lending towards their confusion of their own field with ethics (which is the prescriptive study of human behavior; whenever you hear a psychologist saying we have a "duty" to do this or that, or that we "must" do this or that, i.e. to accept unrepentant homosexuals, you are hearing such a confused psychologist). These are a few of the fundamental errors of modern psychology which must be corrected to reform the field.