1 Samuel 20:4

Then said Jonathan unto David, Whatsoever thy soul desireth, I will even do it for thee.

The promise to do whatever David's soul desired was Jonathan's covenant to be united with David as brethren. It arose because the soul of Jonathan was knit with the soul of David, and Jonathan loved him as his own soul. He gave himself to David in that brotherly union, expressing it by stripping himself of his robe and giving it to David with his garments, his sword, his bow, and his girdle.

The promise was given for David's safety when David came to Jonathan wounded in heart and longing to see him, conscious of his own innocence and asking what he had done that Saul sought his life. At first Jonathan believed Saul had changed his purpose and would do nothing without telling him, especially because God's power had restrained Saul at Ramah, so that further harm to David would be open rebellion against God. David earnestly insisted under oath, as the Lord lives, that there was but a step between him and death.

Jonathan then carried out his promise by agreeing to a test at the new moon festival, in which David would be absent to visit Bethlehem and hide three days in the field, while Jonathan would say he had gone home for his family's sacrifice and would watch Saul. If Saul said it was well, David might safely return to court, but if Saul became enraged it would prove David must flee. Through this friendship God in providence preserved the life of Israel's future ruler when Saul sought to kill him.