How does the Bible view swearing as a guarantee of truth?

Genesis 21:23-24“Swear to me in God’s name that you will never deceive me, my children, or any of my descendants. I have been loyal to you, so now swear that you will be loyal to me and to this country where you are living as a foreigner.” Abraham replied, “Yes, I swear to it!”

Exodus 20:7“You must not misuse the name of the Lord your God. The Lord will not let you go unpunished if you misuse his name.”

Leviticus 19:12“Do not bring shame on the name of your God by using it to swear falsely. I am the Lord.”

In the Old Testament the use of God’s name was the ultimate guarantee of truth-telling. It would be a deadly error to claim God’s name to support what was false. God’s name was treated with such respect that eventually Jewish people avoided saying it aloud, using instead a descriptive term like “Lord.”

Matthew 5:33-37“You have also heard that our ancestors were told, ‘You must not break your vows; you must carry out the vows you make to the Lord.’ But I say, do not make any vows! Do not say, ‘By heaven!’ because heaven is God’s throne. And do not say, ‘By the earth!’ because the earth is his footstool. And do not say, ‘By Jerusalem!’ for Jerusalem is the city of the great King. Do not even say, ‘By my head!’ for you can’t turn one hair white or black. Just say a simple, ‘Yes, I will,’ or ‘No, I won’t.’ Anything beyond this is from the evil one.”

James 5:12Most of all, my brothers and sisters, never take an oath, by heaven or earth or anything else. Just say a simple yes or no, so that you will not sin and be condemned.

Both Jesus and his half-brother James taught that swearing was a vain attempt to add veracity to what should be as truthful a statement as a human could make.

Matthew 23:16-22“Blind guides! What sorrow awaits you! For you say that it means nothing to swear ‘by God’s Temple,’ but that it is binding to swear ‘by the gold in the Temple.’ Blind fools! Which is more important—the gold or the Temple that makes the gold sacred? And you say that to swear ‘by the altar’ is not binding, but to swear ‘by the gifts on the altar’ is binding. How blind! For which is more important—the gift on the altar or the altar that makes the gift sacred? When you swear ‘by the altar,’ you are swearing by it and by everything on it. And when you swear ‘by the Temple,’ you are swearing by it and by God, who lives in it. And when you swear ‘by heaven,’ you are swearing by the throne of God and by God, who sits on the throne.”

This extended scathing criticism by Jesus was directed at those who wanted to play verbal games with the truth. Their splitting hairs over the strength or weight of certain oaths over other oaths revealed their lack of true value. If they were actually telling the truth, they wouldn’t need to use an oath at all to make it sound more convincing.