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.
I write this to you who believe in the Son of God, so that you may know you have eternal life.