Salvation is belief in the Son of God alone
The one who believes in the Son of God has this testimony within himself. The one who does not believe God has made him a liar, because he has not believed in the testimony God has given about his Son. And this is the testimony: God has given us eternal life, and this life is in his Son. 1 John 5:10-11 CSB
Throughout the epistle John uses contrast and our text is no exception. First, John states the positive and then the negative. Belief in the Son of God is central; it is part of the message John teaches in verses 1–12, namely, faith in Jesus as the Son of God. Believing, says John, is a continuous act. That is, faith is a lasting and active power that resides in the heart of the believer. Faith is the constant bond between the Son of God and the believer. The preposition “in” means that the believer puts full trust and confidence in Jesus Christ, the Son of God. The believer has accepted the testimony (see John 3:33; Romans 8:16) which God, through the Spirit, has given about his Son. And this testimony which comes to him through external witnesses is now lodged in his heart and has become an integral part of his spiritual life. So John continuing this theme of testimonies from the two previous devotionals, now declares another hard line in the sand: one either believes that God’s testimony of the Son is true or false, “the one who does not believe God has made Him a liar, because He has not believed in the testimony God has given about His Son.” If human testimony is trusted so much due to witnesses being present, how much more should God, the Almighty Creator, and His testimony be held in high regard? Then, John elaborates what this testimony exactly is: “God has given us eternal life, and this life is in His Son.” This declaration was most likely written to address the antichrists who sowed seeds of doubt in the local church believer’s hearts. As a reminder again, John was writing this letter as a form of encouragement to the believers to reassure them of their salvation, as they were under attack from these false teachers within. By John establishing this hard line and key truth of salvation, the believers are reminded of their salvation and can be confident in what they believe to be true: Jesus Christ is the Son of God and through His death and resurrection has given them eternal life, a truth found in John 3:16, “For God so loved the world, that He gave His One and only Son, that whoever believes in Him shall not perish, but have eternal life.” So, for us, we can know and have confidence in salvation if we have made the decision to follow Jesus and believe in Him, but if you have not yet done so, you are in direct opposition to God. There is no middle ground.
Isaac De Guzman