1 Samuel 20:15
But also thou shalt not cut off thy kindness from my house for ever: no, not when the LORD hath cut off the enemies of David every one from the face of the earth. But also thou shalt not cut off thy kindness from my house for ever: no, not when the LORD hath cut off the enemies of David every one from the face of the earth.
The covenant belongs to the time when Jonathan and David were united as brothers after David killed Goliath, when Saul kept David with him and would not let him return to his father's house. At that time the soul of Jonathan was knit to the soul of David and Jonathan loved David as his own soul, showing it by stripping himself of his robe and giving David his robe with his garments, his sword, his bow, and his girdle. In that period David was entrusted with important responsibilities yet preserved his modesty and won the affection of both the people and the royal household. The verse records Jonathan asking David not to cut off loyal kindness from Jonathan's house forever, even when the Lord has cut off every one of David's enemies from the face of the earth. The covenant of friendship therefore reaches beyond their own lives to include their descendants, and Jonathan's final promise to David renews that lasting bond.
The sacred bond between Jonathan and David endured through Saul's death, and God used Jonathan to help preserve David's life while preparing David to rule Israel. David stayed mindful of that covenant in his prosperity and fulfilled it by seeking out anyone left of the house of Saul for Jonathan's sake. He fulfilled it toward Jonathan's son Mephibosheth, who had been lame from childhood when his nurse let him fall while fleeing at Saul's defeat by the Philistines at Jezreel. David summoned him to court, restored Saul's private possessions to him for the support of his household, and made him his constant guest sitting daily at the royal table.
Though Mephibosheth had been led by reports from David's enemies to cherish prejudice against David as a usurper, David's generous and courteous reception and continued kindness won his heart. So like his father Jonathan he became strongly attached to David and one in interest with the king God had chosen.