And Jesus Christ was revealed as God's Son by his baptism in water and by shedding his blood on the cross -- not by water only, but by water and blood. And the Spirit also gives us the testimony that this is true.
References for 1 John 5:6
So we have these three witnesses --
References for 1 John 5:7
the Spirit, the water, and the blood -- and all three agree.
Since we believe human testimony, surely we can believe the testimony that comes from God. And God has testified about his Son.
All who believe in the Son of God know that this is true. Those who don't believe this are actually calling God a liar because they don't believe what God has testified about his Son.
And this is what God has testified: He has given us eternal life, and this life is in his Son.
So whoever has God's Son has life; whoever does not have his Son does not have life.