What does Psalm 55:21 mean?

The danger David faced (Psalm 55:2–8) was partly enabled by a traitorous former ally (Psalm 55:12–13, 20). This was most likely Ahithophel, the advisor who sided with David's rebellious son, Absalom (2 Samuel 15:12, 14, 31). Absalom's rebellion took time to develop as he won over more and more people in Israel. It is highly likely that Ahithophel sided with Absalom long before making that choice public. Only afterwards would David have realized his counselor had been lying.

Here, David poetically describes that two-faced approach. When he was David's counselor, Ahithophel would have spoken careful, supportive words. Yet, there was malice inside. It's possible Ahithophel was angered by David's treatment of Bathsheba. She was illegitimately taken by David (2 Samuel 12:9), and probably Ahithophel's granddaughter (2 Samuel 11:3; 15:12; 23:34).

Jesus warned His followers to beware of evil intent hidden behind comforting appearances. He said false prophets "come to you in sheep's clothing but inwardly are ravenous wolves" (Matthew 7:15). More colloquially, they are "wolves in sheep's clothing." False teachers often use flattery to capture the minds and hearts of those who are not alert. In the early Christian church, the Galatians were listening to persuasive talkers who presented a false gospel (Galatians 1:6–9). Apparently, the false teachers were hoping to win human favor (Galatians 4:17); whereas Paul was only concerned about gaining the Lord's approval (Galatians 1:10). Paul warned Timothy of those who have "the appearance of godliness, but [deny] its power" (2 Timothy 3:5) and of false teachers "who creep into households" (2 Timothy 3:6). Peter warned of false teachers "who will secretly bring in destructive heresies, even denying the Master who bought them … And in their greed they will exploit you with false words" (2 Peter 2:1–3). Jude, likewise, spoke of those who "have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ" (Jude 1:4). Those who cause great harm to others are not always readily apparent. But God sees the heart and we can entrust ourselves to Him, standing firm in His truth so that we will not be easily deceived.