What does it mean, then, to be a Christian? It means to continue to receive and accept the witness of the apostles, the eyewitnesses of our salvation. It means to believe in Christ with the same faith that was born in them from the works and the words of the risen Lord.
This is what the Apostle John writes: "The way we can be sure of our knowledge of him is to keep his commandments. The man who claims, 'I know him,' without keeping his commandments, is a liar; in such a one there is no truth. But whoever keeps his word, truly has the love of God been made perfect in him" (1 Jn. 2:3-5).
The apostle is speaking of a living faith. Faith is living when it bears the fruit of good works. These are the works of love. Faith is alive through God's love in us. Love is expressed in the observance of the commandments. There can be no contradictions between the knowledge of "I know him" and the actions of one who confesses Christ. Only he who completes his faith with good works remains in the truth.
--Blessed John Paul II