Why Do We Say Someone Was Saved on a Particular Date If It Was Part of an Eternal Plan?

Questions about why we say someone was saved on a particular date if it was part of an eternal plan, the Roman Catholic view of the gospel vs. the Bible’s, and why Paul circumcised Timothy but rebuked the Galatians for saying Gentiles needed to be circumcised. Since God had a plan from all eternity to redeem a people for himself, why do we say someone was saved on a particular date or moment in time? Can you suggest some tactical questions to ask a Roman Catholic relative who doesn’t see the distinction between the Catholic view of the gospel and the one in the Bible? Why did Paul circumcise Timothy but rebuke the Galatians for saying the Gentiles needed to be circumcised?