“Charity, by which God and neighbor are loved, is the most perfect friendship.”

Supplement, Q98, Article 4
Note: This Supplement to the Third Part was compiled after Aquinas's death by Regnald of Piperno, out of material from Aquinas's much earlier "Commentary on the Sentences".
Summa Theologica (1265–1274)
Context: Even as in the blessed in heaven there will be most perfect charity, so in the damned there will be the most perfect hate. Wherefore as the saints will rejoice in all goods, so will the damned grieve for all goods. Consequently the sight of the happiness of the saints will give them very great pain; hence it is written (Isaiah 26:11): "Let the envious people see and be confounded, and let fire devour Thy enemies." Therefore they will wish all the good were damned.

Source: Take a Girl Like You (1960), Ch. 17

Fifth Mansion, Ch. 3, translated by the Benedictines of Stanbrook (1921), revised and edited by Fr. Benedict Zimmerman (1930); reprinted (2003) by Kessinger Publications, p. 109
Interior Castle (1577)
Context: We cannot know whether we love God, although there may be strong reason for thinking so; but there can be no doubt about whether we love our neighbor or not. Be sure that, in proportion as you advance in fraternal charity, you are increasing your love of God, for His Majesty bears so tender an affection for us that I cannot doubt He will repay our love for others by augmenting, and in a thousand different ways, that which we bear for Him.

The Four Cardinal Virtues: Prudence, Justice, Fortitude, Temperance (1965)

Source: The Wisdom of Tenderness: What Happens When God's Fierce Mercy Transforms Our Lives

Source: As You Like It

#64
Vectors: Aphorisms and Ten Second Essays (2001)

Consistory of Cardinals: Exclusive Interview with Monsignor Felipe Arizmendi https://zenit.org/2020/11/27/consistory-of-cardinals-exclusive-interview-with-monsignor-felipe-arizmendi/ (27 November 2020)

Source: Lumina and New Lumina (1969), p. 15