The Lord enjoins us to do good to all without exception, though the greater part, if estimated by their own merit, are most unworthy of it. But Scripture subjoins a most excellent reason, when it tells us that we are not to look to what men in themselves deserve, but to attend to the image of God, which exists in all, and to which we owe all honour and love. … Say that he is unworthy of your least exertion on his account; but the image of God, by which he is recommended to you, is worthy of yourself and all your exertions. … We are not to reflect on the wickedness of men, but look to the image of God in them, an image which, covering and obliterating their faults, should by its beauty and dignity allure us to love and embrace them.
John Calvin, Institutes of the Christian Religion, book III, chapter 7, section 6.