He that hateth dissembleth with his lips, and layeth up deceit within him;
Not every friend loves you. Some who say they love you are hiding hatred in wicked hearts. They dissemble, or pretend, to be your friends, and they lie to cover their evil thoughts and ambitions. But wise men learn this terrible deceitfulness and wickedness of human nature, and they protect themselves from believing fair words too deeply.
The larger context here is a 12-verse description of how wicked men destroy people and relationships (Pr 26:16-28). Solomon condemned meddling, deceitful jesting, talebearing, evil speaking, ensnaring, lying, and flattering. Because their deeds are so cruel, these wicked men generally hide their intentions under gestures of friendship and kind words.
The smaller context describes lying hypocrites, who pretend to be friends, but they have seven abominations in their hearts. The lesson is to not believe all you hear, especially flattery or words of affection (Pr 26:25; 14:15; Jer 9:8; 12:6; Mic 7:1-6). For those who trust God and follow His inspired wisdom, He will expose these liars and their hatred (Pr 26:26).
Only ignorant fools think men and women have good and honest hearts by nature. God has plainly declared that human hearts are desperately wicked (Jer 17:9). Their throats, tongues, lips, and mouths are all cruel and deadly (Rom 3:13-14). This is typical of ungodly men: "The words of his mouth were smoother than butter, but war was in his heart: his words were softer than oil, yet were they drawn swords" (Ps 55:21).
Consider Bible examples. Cain talked kindly to Abel, before killing him (Gen 4:8)! Simeon and Levi arranged a marriage, but only to slaughter a city (Gen 34:6-31)! Joseph's brothers comforted their father about Joseph, after selling him as a slave (Gen 37:31-36)! King Saul flattered David and offered his daughters, but sought to destroy him (I Sam 18:17-29)! Joab feigned kindness to Abner, but only to murder him (II Sam 3:27)!
Absalom appeared content with Amnon, but only to kill him (II Sam 13:22-29)! Absalom flattered Israel, but only for sedition (II Sam 15:1-6)! Joab asked Amasa of his health, but only to kill him (II Sam 20:9-10)! Herod spoke of worshipping Jesus, but had murder in his heart (Matt 2:1-18)! Judas kissed the Lord, but only to betray Him (Matt 26:47-50)!
Wise men learn to be deaf to flattery and praise and attentive and appreciative to rebuke (Pr 2:16; 6:24; 7:5; 20:19; 29:5). They know open rebuke is better than secret love (Pr 27:5; 28:23). They prefer the wounds of a friend than the kisses of an enemy (Pr 27:6; Ps 141:5). They know there is little profit in receiving praise, and it can lead to much harm (Pr 26:28).
Measure all men by the fear of the Lord. Where there is little or no fear of God visible, you should not fully trust that person. The rule is easy to grasp and to apply. Nehemiah wisely rejected Sanballat's invitation to a meeting (Neh 6:1-4). But foolish Gedaliah and other saints believed the lies and tears of Ishmael to their ruin (Jer 40:7-16; 41:1-7)
Solomon wrote this proverb to warn about the hidden dangers of flattery and hypocrisy in relationships, but the blessed God of heaven will expose deceitful and vicious thoughts and intentions (Pr 26:25-26). He alone knows the heart, and He knows all the heart (Jer 17:10; Heb 4:13). Patiently practice wisdom, and leave discovery and vengeance to Him!
Not only will He expose such duplicity and dissimulation, He will also cast the wicked persons into the trouble they had planned for others (Pr 26:27). Absalom was killed cruelly by darts while hanging in a tree (II Sam 18:9-15)! Haman's gallows were used to stretch his neck and those of his ten sons (Esther 7:10; 9:13-14). Daniel's adversaries were eaten by lions, which had slept peacefully with Daniel the night before (Dan 6:21-24)!
The most subtle deceit and cover for hatred is that used by whores, who use flattery to disarm and seduce their victims (Pr 2:16; 5:3; 6:24; 7:5,21; 22:14). The Church of Rome is called a great whore, for she uses religious lies and pretensions to ensnare her victims (Rev 17:1-6; I Tim 4:1-3; Matt 23:14). Do not believe the carefully worded lies that Rome wants to be at peace with other denominations and religions. She does not!