My professor used to say all the time after pronouncing some truth, “It’s just that simple.” Well, salvation and eternal life and the assurance thereof boils down to whether or not you have the Son. Now you may wonder if you’ve got the Son. John has been trying throughout this letter to provide convincing proof to the children of God that indeed they have him. Mostly this comes from the desire to do God’s will, as evidenced by keeping his commandments.
2 By this we know that we love the children of God, when we love God and observe His commandments. 1 John 5:2
Who would desire to do them without possessing the spirit of God? This desire to please God is why God proclaimed his pleasure in his son upon his baptism on earth.
21 Now when all the people were baptized, Jesus was also baptized, and while He was praying, heaven was opened, 22 and the Holy Spirit descended upon Him in bodily form like a dove, and a voice came out of heaven, “You are My beloved Son, in You I am well-pleased.” Luke 3:21-22
Practically, we can testify to all that we believe through our baptism, and remember why our salvation is possible by taking the sacraments. But apart from these the spirit continually breathes into us the assurance that we are his, and tells us regularly that we have the Son, and therefore we have the life. He has convincing proof in pointing to our spirit-motivated actions. But even more so our deep-down desire to please God.
It’s just that simple.
6 This is the One who came by water and blood, Jesus Christ; not with the water only, but with the water and with the blood. It is the Spirit who testifies, because the Spirit is the truth. 7 For there are three that testify: 8 the Spirit and the water and the blood; and the three are in agreement. 9 If we receive the testimony of men, the testimony of God is greater; for the testimony of God is this, that He has testified concerning His Son. 10 The one who believes in the Son of God has the testimony in himself; the one who does not believe God has made Him a liar, because he has not believed in the testimony that God has given concerning His Son. 11 And the testimony is this, that God has given us eternal life, and this life is in His Son. 12 He who has the Son has the life; he who does not have the Son of God does not have the life. 1 John 5:6-12