True, some of them were unfaithful; but just because they were unfaithful, does that mean God will be unfaithful? Certainly not! Romans 3:3-4

“If we are faithless, He remains faithful, for He cannot deny Himself. God is not a man, that he should lie; neither the son of man, that he should repent. Hath he said, and shall he not do it? Or, hath he spoken, and shall he not make it good? Let God be true and every man a liar.” Praise the Lord!