2 Let us see then what it is to believe in Christ; what to believe that Jesus, He is the Christ. He proceeds: “Whosoever believes that Jesus is the Christ, is born of God.” But what is it to believe that? “And every one that loves Him that begot Him, loves Him also that is begotten of Him.” To faith he has straightway joined love, because faith without love is nothing worth. With love, the faith of a Christian; without love, the faith of a devil: but those who believe not, are worse than devils, more stupid than devils.
Some man will not believe in Christ: so far, he is not even upon a par with devils. A person does now believe in Christ, but hates Christ: he has the confession of faith in the fear of punishment, not in love of the crown: thus the devils also feared to be punished. Add to this faith love, that it may become a faith such as the Apostle Paul speaks of, a “faith which works by love:” you have found a Christian, found a citizen of Jerusalem, found a fellow-citizen of the angels, found a pilgrim sighing in the way: join yourself to him, he is your fellow-traveller, run with him, if indeed you also art this.
“Every one that loves Him that begot Him, loves Him also that is begotten of Him.” Who “begot”? The Father. Who “is begotten”? The Son. What says he then? “Every one that loves the Father, loves the Son.”
Source: Homilies on the First Epistle of John (New Advent)