What does Psalm 41:9 mean?

This psalm expresses David's confidence that God will "heal" him by forgiving his sin (Psalm 41:1–4) and giving him victory over his enemies (Psalm 41:5–8). Most likely, this relates to the rebellion of David's son, Absalom (2 Samuel 15—18). This attempted takeover of the kingdom involved plotting and conspiracies, placing David in extreme danger.

One of the enemies who worked against David was Ahithophel (2 Samuel 16:15—17:23)—though he was once a trusted advisor (2 Samuel 16:23). This betrayal may have been inspired by David's mistreatment of Bathsheba (2 Samuel 12:7–9). Scripture suggests she was Ahithophel's granddaughter (2 Samuel 11:3; 23:34). Ahithophel planned no mercy for David (2 Samuel 17:1–3); it was only by God's intervention that Ahithophel's shrewd strategy was ignored and David survived (2 Samuel 17:14).

Jesus used this verse to refer to Judas, who betrayed Him. He said, "I am not speaking of all of you; I know whom I have chosen. But the Scripture will be fulfilled, 'He who ate my bread has lifted his heel against me'" (John 13:18). Sharing meals was a sign of friendship and peace in the ancient world. Jesus acknowledged that the betrayal would fulfill Scripture. However, Christ did not include David's reference to trust. Jesus knew all along that Judas was an unbeliever and one of Satan's agents. He said in John 6:70: "Did I not choose you, the twelve? And yet one of you is a devil." Verse 71 confirms that He spoke of Judas Iscariot.