And we know that the Son of God is come: and he hath given us understanding that we may know the true God, and may be in his true Son. This is the true God and life eternal.

“When ye read, "That we may be in His true Son Jesus Christ," think of the "true Son" of God. But this Son ye in no wise think to be the true Son of God, if ye deny Him to be begotten of the substance of the Father. For was He already Son of Man and by gift of God became Son of God, begotten indeed of God, but by grace, not by nature? Or, though not Son of Man, yet was He some sort of creature which, by God's changing it, was converted into Son of God? If you mean nothing of this sort, then was He either begotten of nothing, or of some substance. But thou hast relieved us from all fear of having to suppose that you affirm the Son of God to be of nothing, for thou hast declared that this is not your meaning. Therefore, He is of some substance. If not of the substance of the Father, then of what? Tell me. But ye cannot find any other. Consequently, the Father and the Son are of one and the same substance. This is the Homousion. In the Scriptures both you and We read, "That we may be in His true Son Jesus Christ; He is the true God and Eternal Life." Let both parties yield to such weighty evidence. Tell us then, whether this "true Son" of God, discriminated as He is by the property of this name from those who are sons by grace, be of no substance or of some substance. Thou sayest, "I do not say that He is of no substance, lest I should say that He is of nothing." He is therefore of some substance: I ask, of what? If not of the substance of the Father, seek another. If thou findest not another, as indeed thou canst find none at all, then acknowledge it to be the Father's, and confess the Son Homousios, "of one substance with the Father." Flesh is begotten of flesh, the Son of flesh is begotten of the substance of the flesh. Set aside corruption, reject from the eye of the mind all carnal passions, and behold "the invisible things of God understood by the means of the things that are made." Believe that the Creator who hath given flesh power to beget flesh, who hath given parents power of the substance of the flesh to generate "true sons" of flesh, much more had power to beget a "true Son" of His own substance, and to have one substance with the true Son, the spiritual incorruption remaining and carnal corruption being altogether alien therefrom. If He is begotten, He is Son: if He is Son, He is the "true Son," because Only-Begotten. For we also are called sons: He Son by nature, we sons by grace. To say that because He is begotten, He is of another nature, is to deny that He is the "true Son." Now we have the Scripture: "That we may be in His true Son Jesus Christ; He is the true God and Eternal Life." Why "true God"? because "true Son" of God. For if He has given to animals this property, that what they beget shall be none other than what they themselves are: man begets man, dog begets dog, and should God not beget God? If then He is of the same substance, why callest thou Him less? Is it because when a human father begets a son, though human beget human, yet greater begets less? If so, then let us wait for Christ to grow as human beings grow whom human beings beget! But if Christ, ever since He was begotten (and this was not in time but from eternity), is what He is, and yet is less than the Father, at that rate the human condition is the better of the two: for a human being at any rate can grow, and has the property of sooner or later attaining to the age, to the strength of the father; but He never: then how is He a "true Son"? And if the Son be not of the same substance as the Father, then is He a made substance: if a made substance, then not "all things were made by Him:" but, "all things were made by Him;" therefore, He is of one and the same substance with the Father. And therefore, not only God, but True (or, Very) God. Which the same John doth most openly affirm in his epistle: "We know that the Son of God is come; and hath given us an understanding that we may learn to know the True God, and may be in His true Son Jesus Christ. This is the True God and Eternal Life." Hence also by consequence we understand, that what the apostle Paul saith, "Who only hath immortality," he saith not merely of the Father, but of the One and Only God, which the Trinity itself is. For neither is the "Eternal Life" itself mortal in respect of any mutability: and consequently, since the Son of God "is Eternal Life," He also is to be understood together with the Father, where it is said, "Who only hath immortality."”

“And we know that the Son of God is come,.... That the second Person in the Godhead, who is equal to the Father, and of the same nature with him, is come from the Father, from heaven into this world, not by local motion, but by assumption of nature; that he is come in the flesh, or is become incarnate, in order to work out salvation for his people, by his obedience, sufferings, and death; and this John and others knew, for they had personal knowledge of him, and converse with him; they saw him with their eyes, heard him, and handled him: he dwelt among them, preached to them, wrought miracles before them, which proved him to be what he was; and it may be known that the Messiah must become, since Daniel's weeks, which fixes the time of his coming, are long ago up; the sceptre is departed from Judah, and the second temple is destroyed, neither of which were to be till the Messiah came; and that Jesus of Nazareth is he who is come may be known by the characters of him, and the works done by him: and hath given us an understanding; not a new faculty of the understanding but new light into it; a knowledge of spiritual things of himself, and of God in him, and of the truths of the Gospel, and of all divine and heavenly things; for he, the Son of God, is come a light into the world, and gives spiritual light to men: that we may know him that is true; or "the true God", as the Alexandrian copy and some others, and the Vulgate Latin, Arabic, and Ethiopic versions read; that is, God the Father, who is the true God, in opposition to the false gods of the Heathens, though not to the exclusion of the Son and Spirit; and the spiritual knowledge of him as the Father of Christ, and as a covenant God and Father in him, is only given to men by Christ, and this is life eternal; see Mat 11:27; and we are in him that is true, even in his Son Jesus Christ; the words "Jesus Christ" are left out in the Alexandrian copy, and in the Vulgate Latin version; however, certain it is, that Jesus Christ is meant by his Son, who is the Son of the true and living God, and is himself "true"; not only true God, as hereafter asserted, but true man, having a true body and a reasonable soul, and was true and faithful in the discharge of his offices, as prophet, priest, and King; he faithfully declared the whole will of God, and taught the way of God in truth; he was faithful to him that appointed him, by securing his glory when he made reconciliation for the sins of the people; and all the administrations of his kingly office are just and true; yea, he is truth itself, the substance of all the types, in whom all the promises are yea and amen, and who has all the truths of the Gospel and treasures of wisdom in him; now his people are in him; they were secretly in him before the world was, being loved by him, chosen in him, put into his hands, preserved in him, and represented by him; and openly, at conversion, when they are anew created in him, brought to believe in him, and live upon him, and he lives in them, and they are in him as branches in the vine; and this is known by his Spirit being given them, by the communication of his grace unto them, and by the communion they have with him. This is the true God and eternal life; that is, the Son of God, who is the immediate antecedent to the relative "this"; he is the true God, with his Father and the Spirit, in distinction from all false, fictitious, or nominal deities; and such as are only by office, or in an improper and figurative sense: Christ is truly and really God, as appears from all the perfections of deity, the fulness of the Godhead being in him; from the divine works of creation and providence being ascribed to him; and from the divine worship that is given him; as well as from the names and titles he goes by, and particularly that of Jehovah, which is incommunicable to a creature; and he is called "eternal life", because it is in him; and he is the giver of it to his people; and that itself will chiefly consist in the enjoyment and vision of him, and in conformity to him.”

“We know that the Son of God is come - In the flesh, and has made his soul an offering for sin; and hath given us an understanding - a more eminent degree of light than we ever enjoyed before; for as he lay in the bosom of the Father, he hath declared him unto us; and he hath besides given us a spiritual understanding, that we may know him who is true, even the True God, and get eternal life from him through his Son, In whom we are by faith, as the branches in the vine, deriving all our knowledge, light, life, love, and fruitfulness from him. And it is through this revelation of Jesus that we know the ever blessed and glorious Trinity; and the Trinity, Father, Word, and Holy Ghost, in the eternal, undivided unity of the ineffable Godhead.”

“Summary of our Christian privileges. is come--is present, having come. "HE IS HERE--all is full of Him--His incarnation, work, and abiding presence, is to us a living fact" [ALFORD]. given us an understanding--Christ's, office is to give the inner spiritual understanding to discern the things of God. that we may know--Some oldest manuscripts read, "(so) that we know." him that is true--God, as opposed to every kind of idol or false god (Jo1 5:21). Jesus, by virtue of His oneness with God, is also "He that is true" (Rev 3:7). even--"we are in the true" God, by virtue of being "in His Son Jesus Christ." This is the true God--"This Jesus Christ (the last-named Person) is the true God" (identifying Him thus with the Father in His attribute, "the only true God," Joh 17:3, primarily attributed to the Father). and eternal life--predicated of the Son of God; ALFORD wrongly says, He was the life, but not eternal life. The Father is indeed eternal life as its source, but the Son also is that eternal life manifested, as the very passage (Jo1 1:2) which ALFORD quotes, proves against him. Compare also Jo1 5:11, Jo1 5:13. Plainly it is as the Mediator of ETERNAL LIFE to us that Christ is here contemplated. The Greek is, "The true God and eternal life is this" Jesus Christ, that is, In believing in Him we believe in the true God, and have eternal life. The Son is called "He that is TRUE," Rev 3:7, as here. This naturally prepares the way for warning against false gods (Jo1 5:21). Jesus Christ is the only "express image of God's person" which is sanctioned, the only true visible manifestation of God. All other representations of God are forbidden as idols. Thus the Epistle closes as it began (Jo1 1:1-2).”