The answer is that assurance is rooted in our election. Second Peter 1:10 says, “Be all the more diligent to confirm your calling and election, for if you practice these qualities you will never fall.” Divine election is the foundation of God’s commitment to save me, and therefore that he will undertake to work in me by sanctifying grace what his electing grace has begun.
This is the meaning of the new covenant. Everyone who believes in Jesus is a secure beneficiary of the new covenant, because Jesus said in Luke 22:20, “This cup that is poured out for you is the new covenant in my blood.” That is, by my blood I secure the new covenant for all who are mine.
Piper, J. (2023). The Bedrock of Assurance