This verse, introduced by γὰρ, shows that the statement about to follow will sustain some portion of the previous one. Which portion? As it seems to me, the coming clause justifies the alteration of the term "the Son" into "the Son of God;" and declares, more fully than any other passage in the New Testament, the lofty and unique character of the Sonship which he claimed.
For even as the Father hath life in himself—the sublime assumption of the self-existence and eternal being of the Father, the absolute Possessor of life per se, the Source ultimate and efficient of all that is connoted by life, the eternal Fountain of life—in like manner also he gave to the Son to have Life in himself.
"He generated," as Augustine has it, "such a Son who should have life in himself, not as a participator in life, but one who should be as he himself is—Life itself." It is the bona fide expression of community of nature, attribute, quality, and possession of Godhead.
In virtue of this utterance, the evangelist, learning from the consciousness of Christ through long years of meditation, under the power of the Spirit, eventually formulated the doctrine of the prologue, "In him was life."
"The Son," or the God-Man, is, so far as this Sonship is concerned, the veritable Son of God with such a fulness of life power and such a fountain of life flowing from him, that his voice is the voice of the Eternal Son.
This is the primary meaning, though since the Lord returned to his use of the word "the Son," and since the word "gave" is also employed to denote the stupendous conception, there is also involved in it the declaration that the God-Man, seeing he is both Son of God and Son of man, is endowed with all the functions of both.
In his incarnation he has not lost the infinite fulness of life giving power. "He quickeneth whom he will," having life in himself. His voice is the voice of the Son of God. The glory of the Word who became flesh was the glory of the Only Begotten.
The part which this great passage took in the Arian controversy is well known (see Athanasius, 'Discourses against Arians,' John 3:3, translated by J.H. Newman). Archdeacon Watkins emphasizes the position that the Lord here speaks of "life in himself," which was given to the Son (God-Man) in virtue of, and as the reward of his sacrificial work.
He points to Philippians 2:6, etc. But Jesus here speaks of a gift already made.