"So God created man in his own image, in the image of God he created him; male and female he created them." (Genesis 1:27 ESV)
"Is it not to share your bread with the hungry and bring the homeless poor into your house; when you see the naked, to cover him, and not to hide yourself from your own flesh?" (Isaiah 58:7 ESV)
"But I say to you, Love your enemies and pray for those who persecute you, so that you may be sons of your Father who is in heaven. For he makes his sun rise on the evil and on the good, and sends rain on the just and on the unjust. " (Matthew 5:44-45 ESV)
"Bless those who persecute you; bless and do not curse them." (Romans 12:14 ESV)
"So then, as we have opportunity, let us do good to everyone, and especially to those who are of the household of faith." (Galatians 6:10 ESV)
Christians are commanded to love all humanity. One reason for this is that there is one human nature, since all are descended from the first human pair (Adam and Eve). The image of God was impressed upon that nature and exists there now, however marred it may be by depravity and sin. The image of God upon all people becomes the basis of our love for all people, no matter how pleasant or unpleasant these people seem to us.
This is a very practical doctrine, as Calvin writes...
"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 honor and love. But in those who are of the household of faith, the same rule is to be more carefully observed, inasmuch as that image is renewed and restored in them by the Spirit of Christ. Therefore, whoever be the man that is presented to you as needing your assistance, you have no ground for declining to give it to him. Say he is a stranger. The Lord has given him a mark which ought to be familiar to you: for which reason he forbids you to despise your own flesh (Gal. 6:10). Say he is mean and of no consideration. The Lord points him out as one whom he has distinguished by the luster of his own image (Isaiah 58:7). Say that you are bound to him by no ties of duty. The Lord has substituted him as it were into his own place, that in him you may recognize the many great obligations under which the Lord has laid you to himself. 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. But if he not only merits no good, but has provoked you by injury and mischief, still this is no good reason why you should not embrace him in love, and visit him with offices of love. He has deserved very differently from me, you will say. But what has the Lord deserved? Whatever injury he has done you, when he enjoins you to forgive him, he certainly means that it should be imputed to himself. In this way only we attain to what is not to say difficult but altogether against nature, to love those that hate us, render good for evil, and blessing for cursing, remembering that 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, III:7:6)