Genesis 17:8 says, "8 And I will give to you and to your offspring after you the land of your sojournings, all the land of Canaan, for an everlasting possession, and I will be their God."
How could God make this promise uncondtionally to ethnic Israel, since unbelieving ethnic Jews will be in hell suffering everlasting destruction? The only Israelites that will enjoy Canaan as an everlasting possession are believing Israelites. And, believing Gentiles will be enjoying it right alongside of them (Matt 5:5; Rev 21:1-3).
Thus, the promise of Canaan as an everlasting possession is infallibly certain, yet it is only certain for Israelites who meet the condition of faith. This promise will certainly be fulfilled because God will work faith in all elect Israelites, guaranteeing that they will inherit the promised blessing along with the believing Gentiles, who are one people in Christ (Eph 2:11-22).