OK, now please tell me what is maturing in the fruit of the Spirit?

A newborn believer is premature in the fruit of the Spirit compared to people you believe has reach the state of sinless.

But what is the difference between them? I only see that both are justified people by their faith. Both are FIT for heaven.

And what else we need, rather than God justify us?

In His love

I don't agree with this. I don't think someone looses salvation just because he/she was overcome by temptation.

I agree.

Do you think their salvation under these circumstances depends on whether or not they receive and experience the gift of repentance?

Jesus demonstrated growing in grace and maturing in the fruits of the Spirit as He progressed from childhood to manhood. As you know, such development does not involve outgrowing sinful habits. Instead, it involves recognizing and resisting sin, self, and Satan. It involves subduing, submitting, and subjecting hereditary tendencies to wrong to a sanctified will and mind. More than this, though, it involves becoming more and more like God (as opposed to becoming less and less like Satan). Newborn believers begin perfect or complete in Christ. And then they spend the rest of eternity maturing in the fruits of the Spirit. The difference between babes in Christ and seasoned saints is maturity not imperfection. Both are fit for heaven. They are justified and sanctified, both of which are requirements of salvation.

1 Peter
2:1 Wherefore laying aside all malice, and all guile, and hypocrisies, and envies, and all evil speakings,
2:2 As newborn babes, desire the sincere milk of the word, that ye may grow thereby:
2:3 If so be ye have tasted that the Lord [is] gracious.
4:1 Forasmuch then as Christ hath suffered for us in the flesh, arm yourselves likewise with the same mind: for he that hath suffered in the flesh hath ceased from sin;
4:2 That he no longer should live the rest of [his] time in the flesh to the lusts of men, but to the will of God.

I don't think they lose salvation by their sin, and then God provides the gift of repentance so that they can return to salvation. I think they lose salvation if and when they don't accept the gift of repentance and forgiveness that God provides.

PS - Nothing I posted above should be taken to mean converted Christians are incapable of neglecting to abide in Jesus and reverting back to sinning. We are not free to sin until we are free from sin.

You seem to be suggesting that sinning does not forfeit salvation, that refusing the gift of repentance is what cancels the privileges of salvation.

Yes, otherwise salvation would be just like a yo-yo, as James said, or like a light switch, as you said. I see it more as an umbrella, and losing salvation as stepping out from under the umbrella by refusing to repent. Christ's righteousness still covers you until you have the opportunity to repent.

Moses would be a good example. Did he lose his salvation when he lost his temper and struck the rock?