1 I Suppose ye remember, those of you who were present yesterday, to what place in the course of this epistle our exposition has reached: namely, “He that loves not his brother whom he sees, how can he love God whom he sees not? And this commandment have we from Him, That he who loves God, love his brother also.” Thus far we discoursed. Let us see then what comes next in order. “Whosoever believes that Jesus is the Christ is born of God.” Who is he that believes not that Jesus is the Christ?
He that does not so live as Christ commanded. For many say, “I believe:” but faith without works saves not. Now the work of faith is Love, as Paul the apostle says, “And faith which works by love.” Your past works indeed, before you believed, were either none, or if they seemed good, were nothing worth. For if they were none, you were as a man without feet, or with sore feet unable to walk: but if they seemed good, before you believed, you ran indeed, but by running aside from the way you went astray instead of coming to the goal.
It is for us, then, both to run, and to run in the way. He that runs aside from the way, runs to no purpose, or rather runs but to toil. He goes the more astray, the more he runs aside from the way. What is the way by which we run? Christ has told us, “I am the Way.” What the home to which we run? “I am the Truth.” By Him you run, to Him you run, in Him you rest. But, that we might run by Him, He reached even unto us: for we were afar off, foreigners in a far country. Not enough that we were in a far country, we were feeble also that we could not stir.
A Physician, He came to the sick: a Way, He extended Himself to them that were in a far country. Let us be saved by Him, let us walk in Him. This it is to “believe that Jesus is the Christ,” as Christians believe, who are not Christians only in name, but in deeds and in life, not as the devils believe. For “the devils also believe and tremble,” as the Scripture tells us. What more could the devils believe, than that they should say, “We know who you are, the Son of God?” What the devils said, the same said Peter also.
When the Lord asked them who He was, and whom did men say that He was, the disciples made answer to Him, “Some say that you are John the Baptist; some, Elias; and others, Jeremias, or one of the prophets. He says unto them, But whom say ye that I am? And Peter answered and said, You are the Christ, the Son of the Living God.” And this he heard from the Lord: “Blessed are you, Simon Barjona; for flesh and blood has not revealed it unto you, but my Father which is in heaven.” See what praises follow this faith.
“You are Peter, and upon this rock I will build my Church.” What means, “Upon this rock I will build my Church”? Upon this faith; upon this that has been said, “You are the Christ, the Son of the Living God. Upon this rock,” says He, “I will build my Church.” Mighty praise! So then, Peter says, “You are the Christ, the Son of the Living God:” the devils also say, “We know who you are, the Son of God, the Holy One of God.” This Peter said, this also the devils: the words the same, the mind not the same.
And how is it clear that Peter said this with love? Because a Christian's faith is with love, but a devil's without love. How without love? Peter said this, that he might embrace Christ; the devils said it, that Christ might depart from them. For before they said, “We know who you are, the Son of God,” they said, “What have we to do with you? Why are you come to destroy us before the time?” It is one thing then to confess Christ that you may hold Christ, another thing to confess Christ that you may drive Christ from you.
So then ye see, that in the sense in which he here says, “Whoso believes,” it is a faith of one's own, not as one has a faith in common with many. Therefore, brethren, let none of the heretics say to you, “We also believe.” For to this end have I given you an instance from the case of devils, that you may not rejoice in the words of believing, but search well the deeds of the life.
Source: Homilies on the First Epistle of John (New Advent)