In Christian theology charity, Latin caritas, is by Thomas Aquinas understood as "the friendship of man for God", which "unites us to God". He holds it as "the most excellent of the virtues". Further, Aquinas holds that "the habit of charity extends not only to the love of God, but also to the love of our neighbor".
Some delineate charity to mean only benevolent giving, while others, such as Roman Catholics, have multiple interrelated meanings (i.e. charity is the theological virtue by which we love God above all things for his own sake, and our neighbor as ourselves for the love of God).
In Christian theology charity is the greatest of the three theological virtues. Thomas Aquinas does not simply equate charity to "love", which he holds as a passion, not a virtue; rather, translators use the word "friendship", as stated above.
The phrase Deus caritas est from 1 John 4:8—or Θεὸς ἀγάπη ἐστίν (Theos agapē estin) in the original Greek —is in line with the translation from Douay-Rheims:
"God is charity" (1 John 4:8)
In contrast with the translation from KJV:
"God is love" (1 John 4:8)
Charity is held to be the ultimate perfection of the human spirit, because it is said to both glorify and reflect the nature of God. Confusion can arise from the multiple meanings of the English word "love". The love that is caritas is distinguished by its origin, being divinely infused into the soul, and by its residing in the will rather than emotions, regardless of what emotions it stirs up. According to Aquinas, charity is an absolute requirement for happiness, which he holds as man's last goal.
Charity has two parts: love of God and love of man, which includes both love of one's neighbor and one's self.
Paul describes it in the First Letter to the Corinthians, chapter 13 (KJV):
Though I speak with the tongues of men and of angels, and have not charity, I am become as sounding brass, or a tinkling cymbal. And though I have the gift of prophecy, and understand all mysteries, and all knowledge; and though I have all faith, so that I could remove mountains, and have not charity, I am nothing. And though I bestow all my goods to feed the poor, and though I give my body to be burned, and have not charity, it profiteth me nothing.
Charity never faileth: but whether there be prophecies, they shall fail; whether there be tongues, they shall cease; whether there be knowledge, it shall vanish away. For we know in part, and we prophesy in part. But when that which is perfect is come, then that which is in part shall be done away.
When I was a child, I spoke as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things. For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.
And now abideth faith, hope, charity, these three; but the greatest of these is charity.
Note that the King James Version uses both the words charity and love to translate the idea of caritas / ἀγάπη (agapē): sometimes it uses one, then sometimes the other, for the same concept. Most other English translations, both before and since, do not; instead throughout they use the same more direct English word love, so that the unity of the teaching should not be in doubt. Love can have other meanings in English, but as used in the New Testament it almost always refers to the virtue of caritas.
Many times when charity is mentioned in English-language bibles, it refers to "love of God", which is a spiritual love that is extended from man to God. One example is "charity shall cover the multitude of sins" (1 Peter 4:8), which forms the basis of perfect contrition.