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.
"Interior Castle" by Teresa of Avila, (Ch. 3), translated by the Benedictines of Stanbrook (1921), revised and edited by Fr. Benedict Zimmerman, 1577.
