Jesus - the Divine Christ! He experienced a life-giving birth and a death-killing death. Not only birth from the womb, but baptismal birth of his ministry and sacrificial death. And all the while the Spirit is confirming the truth, the reality of God's presence at Jesus' baptism and crucifixion, bringing those occasions alive for us.[(s15.7)] A triple testimony:
the Spirit, the Baptism, the Crucifixion. And the three in perfect agreement.
If we take human testimony at face value, how much more should we be reassured when God gives testimony as he does here, testifying concerning his Son.
Whoever believes in the Son of God inwardly confirms God's testimony. Whoever refuses to believe in effect calls God a liar, refusing to believe God's own testimony regarding his Son.
This is the testimony in essence: God gave us eternal life; the life is in his Son.
So, whoever has the Son, has life; whoever rejects the Son, rejects life.