Happiness in life is to be gained by living virtuously, inasmuch as thus a Christian, whilst despising glory and the favour of men, desires to please God alone in what he does.
1. In the first book we spoke of the duties which we thought befitted a virtuous life, whereon no one has ever doubted but that a blessed life, which the Scripture calls eternal life, depends. So great is the splendour of a virtuous life that a peaceful conscience and a calm innocence work out a happy life. And as the risen sun hides the globe of the moon and the light of the stars, so the brightness of a virtuous life, where it glitters in true pure glory, casts into the shade all other things, which, according to the desires of the body, are considered to be good, or are reckoned in the eyes of the world to be great and noble.
2. Blessed, plainly, is that life which is not valued at the estimation of outsiders, but is known, as judge of itself, by its own inner feelings. It needs no popular opinion as its reward in any way; nor has it any fear of punishments. Thus the less it strives for glory, the more it rises above it. For to those who seek for glory, that reward in the shape of present things is but a shadow of future ones, and is a hindrance to eternal life, as it is written in the Scriptures: “Verily, I say unto you, they have received their reward.” This is said of those who, as it were, with the sound of a trumpet desire to make known to all the world the liberality they exercise towards the poor. It is the same, too, in the case of fasting, which is done but for outward show. “They have,” he says, “their reward.”
3. It therefore belongs to a virtuous life to show mercy and to fast in secret; that you may seem to be seeking a reward from your God alone, and not from men. For he who seeks it from man has his reward, but he who seeks it from God has eternal life, which none can give but the Lord of Eternity, as it is said: “Verily, I say unto you, today shall you be with Me in Paradise.” Wherefore the Scripture plainly has called that life which is blessed, eternal life. It has not been left to be appraised according to man's ideas on the subject, but has been entrusted to the divine judgment.
Source: On the Duties of the Clergy (New Advent)