So when God desired to show more convincingly to the heirs of the promise the unchangeable character of his purpose, he guaranteed it with an oath, so that by two unchangeable things, in which it is impossible for God to lie, we who have fled for refuge might have strong encouragement to hold fast to the hope set before us. Hebrews 6:17–18 (ESV)
What he bought was not the nullification of our wills as though we didn’t have to hold fast, but the empowering transformation of our wills so that we want to hold fast.
John Piper, Hold Fast to Your Hope
Semper Reformanda 