The Lord tells us: I am the light of the world; he who follows me will not walk in darkness, but will have the light of life. In these few words he gives a command and makes a promise. Let us do what he commands so that we may not blush to covet what he promises and to hear him say on the day of judgment: “I laid down certain conditions for obtaining my promises. Have you fulfilled them?” If you say: “What did you command, Lord our God?” he will tell you: “I commanded you to follow me. You asked for advice on how to enter into life. What life, if not the life about which it is written: With you is the fountain of life?” Let us do now what he commands. Let us follow in the footsteps of the Lord. Let us throw off the chains that prevent us from following him. Who can throw off these shackles without the aid of the one addressed in these words: You have broken my chains? Another psalm says of him: The Lord frees those in chains, the Lord raises up the downcast.
From a treatise on John by Saint Augustine, bishop