In 1 Samuel, David and Saul's son Jonathan make a sacred covenant. What type of covenant was it?
By the time David finished speaking with Saul, Jonathan’s life became bound up with David’s life; he loved him as his very self. Saul retained David on that day and did not allow him to return to his father’s house. Jonathan and David made a covenant, because Jonathan loved him as his very self. Jonathan took off the cloak he was wearing and handed it over to David, along with his military dress, even his sword, bow, and belt (1 Sam. 18:1-4).
The exact nature of the covenant is not explained. The fact that Jonathan hands over his clothing and weapon, which as the king's son and a military commander would have been royal clothing and a royal weapon, is generally interpreted as a sign that Jonathan is ready to hand over the future throne to David. The general idea seems to be that Jonathan knows that David is a potential rival to the throne after Saul dies. Jonathan's actions symbolize that he does not care if David is the one to ascend to the throne instead of him; he values David's friendship too much to fight over the throne. This appears to be a covenant to mutually support one another as close friends, no matter what happens.
We see this arise when Saul plots to kill David (1 Sam. 20). Saul, knowing Jonathan's affection for David, conceals his plan from his son. When David confronts Jonathan about the plot, Jonathan pleads his ignorance and promises to find out the truth and warn David if his life is truly in danger. For upholding his half of the covenant, Jonathan is rewarded by never having his family and descendants cast off as enemies of David when he ascends to the throne.