“Habere non potest Deum patrem qui ecclesiam non habet matrem.”
— No one can have God for his Father, who has not the Church for his mother.
“For the helmsman is recognized in the tempest; in the warfare the soldier is proved.”
— Treatise VII - On the Mortality
“It is a persistent evil to persecute a man who belongs to the grace of God. It is a calamity without remedy to hate the happy.”
— Treatise on Jealousy and Envy ch. ix
“Men imitate the gods whom they adore, and to such miserable beings their crimes become their religion.”
— Letter 1 Letter to Donatus, viii
“Think not that you are thus maintaining the Gospel of Christ when you separate yourselves from the flock of Christ.”
— Letter 43 "To the Roman Confessors, that they should return to unity"
“Humi iacent fessa laboribus viscera, sed poena non est cum Christo iacere. Squalent sine balneis membra situ et sorde deformia, sed spiritaliter intus abluitur quod foris carnaliter sordidatur.”
— The frame wearied with labours lies prostrate on the ground, but it is no penalty to lie down with Christ. Your limbs unbathed, are foul and disfigured with filth and dirt; but within they are spiritually cleansed, although without the flesh is defiled.

Quotes via Wikiquote (CC BY-SA), each with its original source.