Its unity consists rather in the fact that all its thoughts revolve around one great central truth, the incarnation of the Son of God in the person of Jesus Christ for the salvation of the world.  With this truth he begins, and he affirms it authoritatively, as one of the primitive apostolic witnesses:  “That which we have seen and heard declare we unto you.”  Chaps. 1:3; 4:6.  He guards it also against perversion, when he insists upon the reality of our Lord’s incarnation:  “Every spirit that confesseth that Jesus Christ is come in the flesh, is of God:  and every spirit that confesseth not that Jesus Christ is come in the flesh, is not of God” (chap. 4:2, 3), words which are with good reason understood as referring to a very ancient form of error, that of the Docet[oe], who maintained that the Son of God had not a real, but only an apparent body.  The reception through faith of this great truth, that the Son of God has come in the flesh for man’s salvation, brings us into blissful union and communion with the Father and the Son, and thus into the possession of sonship and eternal life.  Chaps. 1:3; 3:1, 2; 4:15; 5:1, 13, 20.  The rejection of this truth is the rejection of God’s own testimony concerning his Son (chaps. 2:22; 5:9, 10), and thus the rejection of eternal life; for out of Christ, the Son of God, there is no life (chap. 5:11, 12).  But this reception of Christ is not a matter of mere theoretic belief.  It is a practical coming to the Father and the Son, and a holy union with them.  The proof of such union with God and Christ is likeness to God and obedience to God’s commandments.  They who profess to know God and to be in him, while they walk in darkness and allow themselves in sin, are liars and the truth is not in them.  Chaps. 1:5-7; 2:4-6; 3:5-10, 24; 5:4, 5, 18.  The sum of all God’s attributes is love; and the sum of Christian character is love also.  Chap. 4:16.  But there can be no true love towards God where there is none towards the brethren; and such love must manifest itself “not in word, neither in tongue; but in deed and in truth.”  Chaps. 3:11-18; 4:7-11, 20, 21; 5:1.  He that loves his brother abides in the light; but he that hates him abides in darkness and death.  Chaps. 2:9-11; 3:14, 15.  All believers have an abiding unction of the Spirit, which enables them to distinguish between truth and falsehood, and keeps them from the seductions of the many antichrists that are abroad.  Chap. 2:18-27.  Such true believers, whose hearts are filled with love, are raised above fear, and have confidence in prayer, and may look forward with joyful confidence to the day of judgment.  Chaps. 2:28; 3:18-20; 4:17, 18; 5:14, 15.  These fundamental truths the apostle reiterates in various forms and connections, intermingling with them various admonitions and promises of a more particular character.  He dwells with especial fulness on the evidences of discipleship as manifested in the daily spirit and life.  There is perhaps no part of God’s word so directly available to the anxious inquirer who wishes to know what true religion is, and whether he possesses it.  He who, in humble reliance on the illumination of the divine Spirit, applies to himself this touchstone of Christian character, will know whether he is of God, or of the world that lies in wickedness.