If we receive the witness of men, the witness of God is greater; for this is the witness of God which He has testified of His Son. He who believes in the Son of God has the witness in himself; he who does not believe God has made Him a liar, because he has not believed the testimony that God has given of His Son. And this is the testimony: that God has given us eternal life, and this life is in His Son. He who has the Son has life; he who does not have the Son of God does not have life. These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God. (1 John 5:9-13)
The witness looks back to the anointing of the believer described in 1 John 2:27 and refers either to the Holy Spirit or to the testimony of the Scripture. The last part of the verse suggests that the Scripture is meant. John is contrasting those who accept and those reject what God says. A person who trusts in Jesus possesses the witness, namely, the truth of God (1 John 5:9). The one who rejects God’s testimony is claiming that God is a liar.
God’s witness or testimony is that He has given us eternal life in His Son. Eternal life is not a wage to be earned, but a gift to be received from God (see Rom. 6:23). John clearly states that our relationship with the Son determines whether we possess eternal life.
Whoever believes in God’s Son has eternal life. He is all you need. You don’t need to wait for eternal life, because it begins the moment you believe. You don’t need to work for it, because it is already yours. You don’t need to worry about it, because you have been given eternal life by God himself—and it is guaranteed.
Some people hope that they will receive eternal life. John says we can know we have it. Our certainty is based on God’s promise that He has given us eternal life through His Son. This is true whether you feel close to God or far away from Him. Eternal life is not based on feelings, but on facts. You can know that you have eternal life if you believe God’s truth. If you aren’t sure that you are a Christian, ask yourself: “Have I honestly committed my life to Him as my Savior and Lord?” If so, you know by faith that you are indeed a child of God.
Some assume that the phrase these things refers to the whole book of 1 John and conclude that the way to know that one has eternal life is not only to believe in the Son but also to live a righteous life and love fellow believers. However, the phrase does not refer to the whole book; but to the immediately preceding verses and similar expressions throughout this letter (1 John 5:9–12; 2:1, 12–14, 21, 26; 4:1). In other words, the foundation of assurance of salvation is belief in God’s Word and His Son, of whom the Spirit and Scripture testify (1 John 5:11, 12). Those who trust Christ can know they have eternal life because God says they have it.
References: NKJV Holy Bible, Life Application Bible (NIV), and the Nelson Study Bible.