Hebrews 6:13–14 (ESV): For when God made a promise to Abraham, since he had no one greater by whom to swear, he swore by himself, saying, “Surely I will bless you and multiply you.”
God is indeed the greatest of all, no one is greater. God himself evidences this in his revelation by swearing by himself as a confirmation of his promise. If God had sworn by anything other than himself then God would have put himself in a lower position than the thing he swore by. If God is lower than anything then we cannot rightly call him God. God is himself the ultimate authority, meaning there is nothing to appeal to beyond him. This is, in part, what we mean when we say God is greatest.
“I do swear by this city ˹of Mecca˺” is how the Sahih International translation of the Quran translates 90:1. Allah is the speaker and he is swearing by Mecca. Immediately this is a problem. The one true God, the God of Abraham who swore by himself because there is no one greater to swear by, would never swear by anything other than himself. For Allah to swear by Mecca is for Allah to place himself under Mecca in some way.
This oath undercuts Allah as truly being God. By these words he denies himself as the ultimate authority, the thing by which there is no further appeal. Instead, he swears by his own creation, confusing the order of God, and bearing witness to the falsehood of his deity.
By contrast, the God of Abraham swore by himself because there is no one greater. And the Scriptures continue with the great hope that comes from serving the one true God, the God of the Bible:
Hebrews 6:13–20 (ESV): “For when God made a promise to Abraham, since he had no one greater by whom to swear, he swore by himself, saying, ‘Surely I will bless you and multiply you.’ And thus Abraham, having patiently waited, obtained the promise. For people swear by something greater than themselves, and in all their disputes an oath is final for confirmation. 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. We have this as a sure and steadfast anchor of the soul, a hope that enters into the inner place behind the curtain, where Jesus has gone as a forerunner on our behalf, having become a high priest forever after the order of Melchizedek.”
The promise of God is sure, because he alone is God. It is the anchor for our soul. It is because our God is greatest.