30 And many are the proofs concerning the life eternal. And when we desire to gain this eternal life, the sacred Scriptures suggest to us the ways of gaining it; of which, because of the length of our discourse, the texts we now set before you shall be but few, the rest being left to the search of the diligent. They declare at one time that it is by faith; for it is written, He that believes in the Son has eternal life, and what follows; and again He says Himself, Verily, verily, I say unto you, He that hears My word, and believes Him that sent Me, has eternal life, and the rest.
At another time, it is by the preaching of the Gospel; for He says, that He that reaps receives wages, and gathers fruit unto life eternal. At another time, by martyrdom and confession in Christ's name; for He says, And he that hates his life in this world, shall keep it unto life eternal. And again, by preferring Christ to riches or kindred; And every one that has forsaken brethren, or sisters, and the rest, shall inherit eternal life. Moreover it is by keeping the commandments, You shall not commit adultery, You shall not kill, and the rest which follow; as He answered to him that came to Him, and said, Good Master, what shall I do that I may have eternal life?
But further, it is by departing from evil works, and henceforth serving God; for Paul says, But now being made free from sin, and become servants to God, you have your fruit unto sanctification, and the end eternal life.
Source: Catechetical Lectures (New Advent)