1 John 5:20

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 ( Matthew 11:27 ) ( John 17:3 ) ;

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.