If the O.T. saints and those left behind need to keep the Law/Commandments in order to be saved, (unlike the Church by grace), then how could anyone in the O.T. go to heaven, since then it would be based on good works? Aren’t we all saved by grace? (Ephesians 2:8,9).
For Israel, keeping the commandments was a condition for maintaining their covenant with God (Exodus 19:5-6) which brought them a homeland and enormous earthly blessings. But it was not sufficient to save them. Both Jesus (Matt. 5:20) and Paul (Romans3:20) confirmed this. What saved them was believing that a redeemer would come to pay the penalty for their sins that the sacrifices they offered only set aside temporarily (Hebrews 10:1, 12-14). Therefore they were saved by faith in the Redeemer who would come, just as we are saved by faith in the Redeemer who has come. Without that faith, God’s grace cannot avail either to them or to us. Ephesians 2:8-9 says we are saved by grace, through faith.
For post Church believers keeping the commandments will be required as evidence of their belief that the Lord has saved them (Rev. 14:12). And even when Israel gets the New Covenant, it will include animal sacrifice (Ezekiel 45:13-17). Only the Church is saved by faith alone.