Once you believe Christ died and was resurrected you are saved. Every sin you will ever commit happened after our savior died for those sins. Study the books of John and Hebrew's.
If our future sins are not forgiven, then two things must happen: 1) when we sin in the future, we revert to an unforgiven state, meaning we lose our salvation and must be saved again, and 2) Christ must die again to cover the sins we’ve committed since His last death. Neither of these scenarios is biblical. We are kept by God, and thus our salvation is secure (
John 10:28–30); and Christ died “once for all” (
Hebrews 10:10; cf.
Hebrews 7:25).