Sorry - but the Bible disagrees with you.
First of all - the idea that you DON'T have to repent of future sins is a Protestant invention and completely UNBIBLICAL.
Secondly - the people in Acts 2 hadn't yet repented or been Baptized - so they were NOT justified.
Finally - the NT is filled with warnings for converted, born again Christians NOT to fall back into willful sin or they will LOSE their security.
Romans 11:22
“See, then, the kindness and severity of God: severity toward those who fell, but God's kindness to you, provided you REMAIN in his kindness; otherwise you to will be cut off.”
Paul is warning the faithful to REMAIN in God’s favor or they will lose their salvation. How can they lose what they never had?
Hebrews 10:26-27
“If we sin deliberately AFTER receiving KNOWLEDGE of the truth, there no longer remains sacrifice for sins but a fearful prospect of judgment and a flaming fire that is going to consume the adversaries.”
This is a clear warning that falling away from God will result in the loss of our salvation. The Greek ford for “knowledge” used here is NOT the usual word (oida). This is talking about a full, experiential knowledge (epignosei). This verse is about CHRISTIANS who had an EPIGNOSIS of Christ and who can fall back into darkness and LOSE their salvation by their own doing.
2 Peter 2:20-22
For if they, having escaped the defilements of the world through the KNOWLEDGE of our Lord and savior Jesus Christ, again become entangled and overcome by them, their last condition is worse than their first.
For it would have been better for them not to have KNOWN the way of righteousness than after knowing it to turn back from the holy commandment handed down to them.
Here, Peter illustrates that those who had a full, experiential knowledge (epignosei) of Christ – CHRISTIANS – who can fall back into darkness and LOSE their salvation by their own doing.
Matt. 5:13
You are the salt of the earth. But if salt loses its taste, with what can it be seasoned? It is no longer good for anything but to be thrown out and trampled underfoot.
This one is self-explanatory - even to a blind person . . .
1 Cor. 9:27
"I pummel my body and subdue it, lest after preaching to others I myself should be disqualified."
Paul is saying that he wrestles with his own fleshly desires so that he might not fall back into sin.
2 Peter 3:17
Therefore, dear friends, since you already know this, be on your guard so that you may not be carried away by the error of lawless men and fall from your secure position.
Peter is warning the faithful not to fall back into sin and lawlessness.
1 John 2:24
See that what you have heard from the beginning remains in you. IF it does, you also will REMAIN in the Son and in the Father.
This is an admonition to try to remain faithful.
Rev. 3:5
He who overcomes will, like them, be dressed in white. I will never blot out his name from the book of life, but will acknowledge his name before my Father and his angels.
God cannot blot out a name that was never there in the first place. He is talking about CHRISTIANS who are already saved and how they can LOSE their salvation.
Rev. 22:19
And if anyone takes words away from this book of prophecy, God will take away from him his share in the tree of life and in the holy city, which are described in this book.
How can God “take away” somebody’s “share” of heaven if they never had it to begin with? This is about CHRISTIANS who may or may NOT make it into Heaven.