Everyone who believes that Jesus is the Christ is a child of God. And everyone who loves the Father loves his children, too.
We know we love God's children if we love God and obey his commandments.
Loving God means keeping his commandments, and really, that isn't difficult.
For every child of God defeats this evil world by trusting Christ to give the victory.
And the ones who win this battle against the world are the ones who believe that Jesus is the Son of God.
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.