God is able to forgive the sins of those that are faithful, but God also knows if we harden our hearts and refuse to repent of our sins. The heart makes all the difference.
We can lose our salvation but again it is not one sin. it is a general hardening of heart that turns away from the living God. This hardening results in a callousness to sin, refusal to repent, loss of love, etc. It is a pattern of behavior. Even David, with the seriousness of some of his sins, repented and did not let his heart remain hard. Even Calvin (whose theology i generally disagree with) did not believe future sins were forgiven. Future sins were not forgiven in advance or necessarily automatically forgiven when they occur. The difference is in paid for and forgiven.
A saved person’s future sins are forgiven. An unsaved person’s are not. A person who does not care if they are sinning is not is not saved.