That made us this soul - This very unusual addition to the formula of an oath 1 Samuel 20:3 was intended to strengthen it. By acknowledging that his soul was God’s workmanship Zedekiah also implied his belief in God’s power over it.
Related Commentaries of Jeremiah 38:16
Jeremiah 38:16
16 So Zedekiah the king sware secretly unto Jeremiah, saying, As the LORD liveth, that made us this soul, I will not put thee to death, neither will I give thee into the hand of these men that seek thy life.