II Cor. 5:10 - For we must all appear before the judgment seat of Christ, so that each one may receive good or evil, according to what he has done in the body.
7) There are three that bear record in heaven...
the Father (God), the Word (Jesus) and the Holy Ghost (Spirit of God)
and these three are one.
This means their testimony regarding us is recorded and secured in heaven and can be relied on.
8) And there are three that bear witness in earth...
the Spirit, and the water, and the blood and these three agree in one.
The Holy Spirit bears witness because the Spirit has been given us as a deposit or guarantee here on earth. The water bears witness because our obedience to the public act of baptism also testifies on our behalf. The blood bears witness because it is the blood of Jesus which covers our sins.
They all testify to the same thing... our salvation.
9) if we receive the witness of men, the witness of God is greater...
If we can stand here on earth and believe the witness of mere men on important matters, how much more secure is God's witness on our behalf?
10-11) And this is the record, that God that given to us eternal life, and this life is in his Son.
This is what is on record:
God has given us eternal life. We received it because of Jesus' death and resurrection, and because of our belief in what Jesus did.
12) He that hath the Son hath life; and he that hath not the Son of God hath not life.
13) These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
Jesus = Eternal Life. No Jesus = No Eternal Life.
It's that simple.