Today is the feast of St. Juan de Avila, Doctor of the Church. From a letter on charity:
The best way to possess true charity and understand what it is, is to consider how the Blessed in heaven practise it, because the more closely we imitate them, the more perfect shall we be in that virtue. The love which the saints bear towards God transforms their will, so that it becomes one with His : that is, they can wish, or not wish, only what He does ; because, as St. Denis says, one effect of love is to make the will of those that love one, and God's whole love and will are centred upon His own glory and essence, which is supremely perfect. It therefore follows that the love of the saints is that single-minded affection and will, with which they long, with all their strength, that God may be in Himself as good, and glorious, and adorable as He is. Seeing Him to possess all these perfections, they feel an ineffable joy, which is the fruit of the Holy Ghost. It will give us some idea of what this happiness is, if we consider how a good son rejoices in seeing his father rich, powerful, wise, beloved and respected by all, and honoured by the king. Indeed, some children are so dutiful, that no troubles or misfortunes of their own can destroy the pleasure they feel in their parents' prosperity, which they consider of the first importance. Now if this human joy for a parent be so great, what must be the jubilation of the saints, transformed as they are by heavenly love, at beholding that God is so holy, perfect, and rich in excellence. They see that, as Creator of the universe, He, by one single act of His will, gave all things their beauty and being, and upholds them in existence, so that not a single leaf can rustle in the wind but by His consent. Behold the joy “such as eye hath not seen, nor ear heard, neither hath it entered into the heart of man.” (1. Cor. II. 9.) This, then, as far as it is possible for us to understand it, is the “charity” of the just in heaven. From this overflowing river which gladdens the city of God, comes, as a streamlet, their love for their neighbour.
[St. John of Avila, from Letters of Blessed Juan de Avila, pp. 126-127.]