Bernard of Clairveaux on the Unity of the Trinity and Love
From On Loving God, Ch. XII:
For what preserves the glorious and ineffable Unity of the blessed Trinity, except love? Charity, the law of the Lord, joins the Three Persons into the unity of the Godhead and unites the holy Trinity in the bond of peace. Do not suppose me to imply that charity exists as an accidental quality of Deity; for whatever could be conceived of as wanting in the divine Nature is not God. No, it is the very substance of the Godhead; and my assertion is neither novel nor extraordinary, since St. John says, 'God is love' (I John 4.8). One may therefore say with truth that love is at once God and the gift of God, essential love imparting the quality of love.
This quote caught my attention because it points to the relation between God's being love, and the Holy Trinity. This is an important topic in our witness to Muslims, of course.