'Solemnly' in the Bible

Be strong and brave! You must lead these people in the conquest of this land that I solemnly promised their ancestors I would hand over to them.

So the Lord gave Israel all the land he had solemnly promised to their ancestors, and they conquered it and lived in it.

The Lord made them secure, in fulfillment of all he had solemnly promised their ancestors. None of their enemies could resist them.