For he who eats and drinks in an unworthy manner eats and drinks judgment to himself, not discerning the Lord's body. For this reason many are weak and sick among you, and many sleep. For if we would judge ourselves, we would not be judged. It goes without saying that we cannot depend on such a procedure keeping us from being condemned. God makes no allowance for His people becoming loose in our living, and sinning willfully.
Sin has a way of hardening the heart.
We do not know the point at which God will close the door on His mercy. That is why we are to run to the throne of grace when we sin. No conscientious believer wants to sin. However, as you state, none of us are sinless. God has made gracious provision for forgiveness, and urges us to appropriate it.