One of the treatment of Ocd is CBT(Cognitive Behavioural Theraphy) it means the person that is suffering from this illness should agree with all of his intrusive thoughts. My question is what if a person experiencing an intrusive thought like(what if I am a Christian) and he use CBT treatment then agrees with that thought, is it allowed then if not what should we do?

If one knows that the thought is wrong, then one should not agree with it and say it to himself. This leads to negative consequences even if it may have some therapeutic effect to treat CBT. For these cases, I recommend the Hadiths/Du’as that we have from the Ahlulbayt (a). They help address negative thoughts and OCD.