9 We accept man’s testimony, but God’s testimony is greater because it is the testimony of God, which he has given about his Son. 10 Anyone who believes in the Son of God has this testimony in his heart. Anyone who does not believe God has made him out to be a liar, because he has not believed the testimony God has given about his Son. 11 And this is the testimony: God has given us eternal life, and this life is in his Son. 12 He who has the Son has life; he who does not have the Son of God does not have life.
The apostle John has given his own testimony many times, but here he tells us that while we (most of us) accept a person’s eye witness account, we have a greater testimony available to us. Lots of people say they have to “see it to believe it.” Those are the ones who would doubt John’s testimony regardless. However God gives us His testimony (that Christ is our only Savior) by placing it right in our hearts. This is why it’s impossible to argue with Christians about these truths: we know the Truth in our hearts. Those "see it to believe it" people change their tune when their hearts are changed. It's amazing to witness.
John finishes this passage by explaining exactly what that testimony is: God has given us eternal life through His Son Jesus. Know Jesus, know life. No Jesus, no life.