Think carefully here. After perhaps 2 years of preaching the gospel that Christ gave Peter and the 11, they did not have the slightest idea what the Lord was talking about when He predicted His death. (Luke 18:31-34). Obviously, the "gospel" which they preached was not the "gospel" which Paul later preached or the "gospel" by which we are saved (1 Cor. 15:1-4). The "gospel" which they preached was "the gospel of the kingdom" (Matt.9:35, Luke (:2), NOT "the preaching of the cross.".
That Kingdom gospel will once again be preached, when Israel is once again God's people (beginning with the tribulation). So you see, His promise to be with ISRAEL until the end of time IS TRUE.
Selah. Here is wisdom.