Corpus of Latin Ecclesiastical Writers, Vol. LXII: Ambrose's Exposition of Psalm 118

Such is doctrine without innocence of life. But neither can doctrine itself have a reward, where innocence does not have grace. Yet God said to the sinner: Why do you declare my justices? But let us return to the proposed psalm.

7. Blessed, he says, are the undefiled in the way, who walk in the law of the Lord. Blessed are they that search his testimonies, that seek him with their whole heart. The first verse is moral, the second is mystical. Who says this? The prophet indeed, who was functioning as the mouthpiece of the flesh, and he says this after that psalm in which the passion of our Lord Jesus Christ was described. Therefore, where divine mysteries were revealed to him and he put on the joy of the Lord’s resurrection and tasted the grace of the passion, he saw the congregations of the just, the peoples of the redeemed, the salvation of the lost, the resurrection of the dead, the sanctification of the sacraments, he cried out, saying: Blessed are the undefiled in the way, who walk in the law of the Lord, that is: behold, the cursed earth in Adam began to have blessedness, if, however, it does not abandon the law of the Lord; behold, the man who was previously polluted is undefiled. How precious it is now to guard the commandment of the Lord, how precious also to know the mystery of the commandment itself!

8. But who is undefiled? Not, indeed, he who walks in just any way, but he who walks in Christ. For He Himself said: I am the way. He who walks in this way does not know how to stray, if, however, he never departs from this way. The law is also a way, and therefore, while he is undefiled, he walks in the law of the Lord; nor should he cease to walk in this way, lest he cease to be undefiled. Let him not decline to the right or to the left, let him not stumble, let him not resist, let him not wait, but let him walk, forgetting the things that are behind and stretching forth to those that are before, let him follow to the mark, let him hasten to the prize, let him contend even to the end; for the end of the law is Christ. Many wish to walk in the way, but not to the end. The Jews do not walk to the end, who do not walk to Christ; the Manichean does not walk in the way, who rejects the law, but true faith walks, which both receives the law and acknowledges the fullness of the law.

References: Psalm 49:16; Psalm 118:1-2; John 14:6; Philippians 3:13-14; Romans 10:4.