Matthew 5:20 For I tell you that unless your righteousness surpasses that of the Pharisees and the teachers of the law, you will certainly not enter the kingdom of heaven.
Notice here that John says that God GAVE us eternal live, past tense. We already possess it and it is given only through His Son. And John wrote what he did so that we may KNOW that we have eternal life. God wants us to be sure.
1 John 5:11 And this is the testimony, that God gave us eternal life, and this life is in his Son. 12 Whoever has the Son has life; whoever does not have the Son of God does not have life. 13I write these things to you who believe in the name of the Son of God that you may know that you have eternal life.
Salvation is by faith so that no one can boast. When it's by works, that leaves room for pride and boasting, that one was good enough to do it. Then God doesn't get the glory, man does, and God will not share His glory with another.
Ephesians 2 1 And you were dead in the trespasses and sins 2 in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience— 3among whom we all once lived in the passions of our flesh, carrying out the desires of the body and the mind, and were by nature children of wrath, like the rest of mankind. 4But God, being rich in mercy, because of the great love with which he loved us, 5even when we were dead in our trespasses, made us alive together with Christ— by grace you have been saved— 6and raised us up with him and seated us with him in the heavenly places in Christ Jesus, 7so that in the coming ages he might show the immeasurable riches of his grace in kindness toward us in Christ Jesus.
8For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, 9 not a result of works, so that no one may boast. 10For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand, that we should walk in them.
[metmom] Nor can works ever give any guarantee of salvation because you never know if you've done enough.
You are correct. You're absolutely right.
However, you're not adding in the other side of the equation. The other side of the equation is daily repentance. Then FAITH that the Lord will forgive us for our sins as He has promised to do.
The Lords promise is our guarantee.