Especially Calvin comments:3657 "It is said [in First John 5:20] that Christ ‘hath given us an understanding’…. He shows us in the teaching of the Gospel of what kind is the true God — and also enlightens us by His Spirit….
"‘This is the true God.’ Although the [unitarian] Arians have tried to get round this passage…, we have here a remarkable title to the divinity of Christ…. The Father is the origin of life — but the fountain from which we must draw it, is [the Son] Christ [the Spirit-anointed One]." Viz. the three divine Persons of the One True Triune God.