Hebrews 6:13-20

13-20. The basis of our assurance that perseverance will assuredly bring glory is the doubly-pledged veracity of God as covenanting and as binding himself by oath. But God neither swears nor promises that we shall persevere. He only promises and swears that there shall be no failure of the reward, if we do, through exerted grace and power derived from him, persevere. Note Romans 8:30. The reason why the old Jews were lost, was not because God did not covenant and swear, but because they did not persevere, and make the covenant and oath binding.