“...if by grace, then it cannot be based on works...”
No human can be saved by their own works - Catholics realize this - no Catholic thinks they can save themselves by works without God’s grace.
However, salvation can be lost - the notion that once saved always saved is what is actually at issue here. People do turn away from God by their works and can lose their salvation. 2 Peter 2:21 For it would be better for them not to have known the way of justice than, after they have known it, to turn back from that holy commandement which was delivered to them...”
Keeping the commandments and doing what scripture tells us to do IS a work (are works) by it’s (their) very nature. Scripture shows that if a person (or nation) doesn’t do what God as reaveled by scripture tells them to do, they can be lost. In the OT, Israel constantly turns away from God and is lost in the end with the destruction of Jerusalem in 70 AD. They believed in God - then disobeyed - disobeying is a WORK.
Where Catholics need to get is to realize that even with God's grace, their works do not count for anything towards procuring salvation.
Works are the result of being saved. The evidence of the salvation that already happened in them that the faith they have is the living faith, not the dead mental assent that too many people mistake as genuine faith.
And the works need to flow naturally out of that salvation. Simply adding works to mental assent thinking that it's accomplishing something, is useless. It's a deep deception which too many people have fallen for.
If one is not born again, one cannot see the kingdom of heaven. Jesus said that we MUST be born again. He never said to simply believe and add good works.
You contradict what the very word "grace" means by this statement. We are saved NOT by the righteous deeds that we do but by God's unmerited mercy and grace. You can't have grace AND works. That's like saying you have to pay for a gift someone gives you. If you have to pay for it, it's not a gift. If you have to work for it, it's not grace.
However, salvation can be lost - the notion that once saved always saved is what is actually at issue here. People do turn away from God by their works and can lose their salvation. 2 Peter 2:21 For it would be better for them not to have known the way of justice than, after they have known it, to turn back from that holy commandement which was delivered to them...”
That verse doesn't say what you presume it to say. Knowing the truth of the gospel and then turning from it DOES cause a greater depth of punishment than someone who had never heard the gospel. We are saved "not because of the righteous things we had done, but because of his mercy. He washed away our sins, giving us a new birth and new life through the Holy Spirit." (Titus 3:5) and, "we know that a person is not justified by works of the law but through faith in Jesus Christ, so we also have believed in Christ Jesus, in order to be justified by faith in Christ and not by works of the law, because by works of the law no one will be justified." (Galatians 2:16)
Keeping the commandments and doing what scripture tells us to do IS a work (are works) by it’s (their) very nature. Scripture shows that if a person (or nation) doesn’t do what God as reaveled by scripture tells them to do, they can be lost.
Then you ARE saying it is by our works, or in your example, obeying all God's laws or refraining from breaking any of God's commandments that we get saved - hence we are saved by what WE do and not what Christ did for us. So just exactly where does God's grace fit in your church's gospel.