25 … this mystery, so that you won’t be wise in your own conceits, that a partial hardening has happened to Israel, until the fullness of the Gentiles has come in, 26 and so all Israel will be saved. Even as it is written, “There will come out of Zion the Deliverer, and he will turn away ungodliness from Jacob. 27 This is my covenant to them, when I will take away their sins.
Here Bible says that there is some “mystery” (that is a cipher) in mind of religious Jews. When God will show how to decipher it, He will deliver some message (which is yet ciphered and unknown to us) which will suddenly turn religious Jews into Christians. It looks impossible while we don’t yet know this message but when we see the message it will become clear.
Knowing how easy to turn unbelievers into believers we should not be proud of our faith (“wise in your own conceits”).