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>> “Salvation can’t be by both grace and works.” <<
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Oxymoron.
Grace is what leads us to the works by the writing of the law on our hearts. Salvation comes at the end, and is a product of a lifetime of righteousness resulting therefrom.
Yeshua, and every apostle told us that we must endure to the end to be saved. Paul constantly cautioned his sheep lest they fail to endure and receive that prize of great value. “work out your salvation with fear and trembling...”
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The Bible is clear that we are saved by grace through faith unto good works. (Ephesians 2:8-9) In Romans three and four, Paul must have said salvation is a totally free gift -- not earned by good works -- fifteen times. You must understand that Scripture teaches when we try to add our work to grace, we cancel grace ("grace is no more grace"). He told the Galatians who were trying to be justified by works they had rendered Christ of no effect and fallen from grace. Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace.(Galatians 5:4)
God simply will not accept human works for salvation. That's the message of Scripture from Genesis forward.
And if by grace, then is it no more of works: otherwise grace is no more grace. But if it be of works, then it is no more grace: otherwise work is no more work.--Romans 11:6
For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: 9 Not of works, lest any man should boast. 10 For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them. --Ephesians 2:8-10
Salvation comes at the beginning with the acceptance of Christ. Works flow from the Spirit working within the saved Christian. On our own we can do nothing.