Reassurance that we will get through what we are going through does help. I don't think that has much to do with belief or power as in how you stated your original question. Reassurance is about support and hope, maybe a bit about faith. Not really the same to me as believing something that isn't true or we don't feel is true.

My pdoc once told me he would find a way to help me feel better. I "believed" him, but it really was more about reassurance and hope than belief. It was about support and commitment.