God shows you that the gospel is true. That's called 'faith', the "evidence of things not seen" (Hebrews 11:1 KJV).
That's God's part. And the way he gives you the 'evidence of things not seen' is through the testimony of the Holy Spirit in our heart (1 John 5:6-11).
After that, you have the responsibility to respond to the irrefutable evidence God gives you with your trust. That's your part.
That isn't earning salvation, that's you believing what you heard. The believing you do in response to what God shows you to be true is what justifies you.
Romans 4:5 KJV
"to him who does not work but believes on Him who justifies the ungodly, his faith is accounted for righteousness"
Most people do not place their trust in Jesus in response to the 'evidence of things not seen' given to them by the Holy Spirit. They continue on in their old, unchanged lives and they are lost. Though some of them think they are saved because they think that just knowing the gospel of Jesus and salvation is true saves them. But they never put their trust in what they heard, or they later stop trusting in what they heard, and they remain unchanged and unsaved while they play the church game. The unchanged life is the sign of this kind of 'believer'. They are hypocrites. Their place will be in the place of weeping and gnashing of teeth, not the kingdom they are sure they possess. If they were really trusting in Christ, they would be living in a new life growing in holiness, not living in a life of 'religious' hypocrisy.
Matthew 24:42-51 KJV
42Watch therefore: for ye know not what hour your Lord doth come. 43But know this, that if the goodman of the house had known in what watch the thief would come, he would have watched, and would not have suffered his house to be broken up. 44Therefore be ye also ready: for in such an hour as ye think not the Son of man cometh.
45Who then is a faithful and wise servant, whom his lord hath made ruler over his household, to give them meat in due season? 46Blessed is that servant, whom his lord when he cometh shall find so doing. 47Verily I say unto you, That he shall make him ruler over all his goods. 48But and if that evil servant shall say in his heart, My lord delayeth his coming; 49And shall begin to smite his fellowservants, and to eat and drink with the drunken; 50The lord of that servant shall come in a day when he looketh not for him, and in an hour that he is not aware of, 51And shall cut him asunder, and appoint him his portion with the hypocrites: there shall be weeping and gnashing of teeth."
These people who think they can go back to their old life of sin and unbelief and still be saved because once they are saved they are always saved are fooling themselves. If they don't come back to Christ (or truly trust in him for the first time, which ever is the case) they will go to the place of hypocrites where there will be weeping and the gnashing of teeth. Falling away and thinking you are still saved (assuming you really were)
because you think once saved always saved is true is sweeping many in the church away to their eternal destruction.