We are sinners!
God has a perfect convincing argument. The unbeliever must admit that their sins are as scarlet, the deepest color of red and that they are guilty.
When they repent and believe, God will save them, He will make their sins white as snow.
How does God do this?
We share the gospel of Jesus Christ!
So faith comes from hearing, and hearing by the word of Christ.
…and we let the Holy Spirit do the saving!