The Lord instructs us to do good to all people throughout the entire world, many of whom are unworthy of such good if judged by their own merit. But Scripture comes to our rescue with the best of reasons for doing good to all people. It teaches us not to regard others according to their own merits, but to consider in them the image of God to which we owe both honor and love. But the image of God should be more diligently regarded in those who are of the household of faith, because it has been renewed and restored in them by the Spirit of Christ.
Therefore, you have no cause to evade anyone who stands before you and needs your service. Suppose he’s a stranger. The Lord, however, has stamped him with his own mark that’s familiar to you, and for that reason God forbids you to despise your own flesh. Suppose he is contemptible and worthless. The Lord, however, shows him to be one whom He has condescended to decorate with His own image.
John Calvin, A Little Book on the Christian Life (pgs. 40-41).