Suppose you promise me to do something and suppose I should trust you. Then I have a moral reason not to check whether you did what your promised. Of course, if I have a special responsibility for it, I may also have a moral reason to check. But generally speaking, I think we have an imperfect duty not to check up on people when we should trust them. Moreover, we should trust people unless we have good reason to the contrary. I would be wronging a colleague if, out of the blue, I were to start running his papers through TurnItIn.com to look for plagiarism. Such an action would be a failure to show required trust. It would thus be contrary to collegial love.
Natural love, thus, requires natural faith of us. But our supernatural love for Christ requires supernatural faith of us.