I do not nullify the grace of God, for if righteousness were through the law, then Christ died for no purpose.
If a person can live well enough to gain access to Heaven then Jesus was slaughtered for no reason.
If a person can get to Heaven without Jesus, God was cruel to allow His Son to suffer such violence only to serve as an example of selflessness.
Right standing with God is only available through faith in Jesus by the grace of God.
If you say there is any other way, you nullify the grace of God and elevate human merit to a place of idolatry.