Paul wanted us to know that our salvation has its roots in God’s grace and our faith-filled response to Him, for salvation is the gift of God, and it is,
“not of works, lest anyone should boast” (Ephesians 2:9).
Yes, there are things we do in our salvation, such as having faith, but God is the One who saves us. Without His grace, we could have all the faith in heaven and on earth, but it would not save us.
Why should we boast of our salvation when we were the ones who got ourselves into eternal trouble initially? God wanted to save us and we wanted Him to, so that all the glory goes to Him.