“And herein we know that we are of the truth, and assure our hearts before Him. For if our heart think ill of us, God is greater than our heart, and knows all things. Beloved, if our heart think not ill of us, then have we confidence toward God. And whatsoever we ask, we shall receive of Him, because we keep His commandments, and do in His sight those things that please Him. And this is His commandment, That we should believe in the name of His Son Jesus Christ, and love one another, as He gave us commandment. And he that keeps His commandments shall dwell in Him, and He in him. And herein we know that He abides in us, by the Holy Spirit which He has given us. Dearly beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into this world. In this is known the Spirit of God: every spirit that confesses that Jesus Christ has come in the flesh is of God: and every spirit that confesses not that Jesus Christ has come in the flesh is not of God: and this is the antichrist, of whom you have heard that he should come; and even now already is he in this world.”
Source: Homilies on the First Epistle of John (New Advent)