5 “Who have imagined unrighteousnesses in their heart”....From them free me, from them let Your hand be most powerful to deliver me. For easy is it to avoid open enmities, easy is it to turn aside from an enemy declared and manifest, while iniquity is in his lips as well as his heart; he is a troublesome enemy, he is secret, he is with difficulty avoided, who bears good things in his lips, while in his heart he conceals evil things. “All the day long did they make war.”
What is, “war”? They made for me what I was to fight against all the day. For from thence, from such hearts as these, arises all that the Christian fights against. Be it sedition, be it schism, be it heresy, be it turbulent opposition, it springs not save from these imaginings which were concealed, and while they spoke good words with their lips, “all the day long did they make war.” You hear words of peace, yet making war departs not from their thoughts. For the words, “all the day long,” signify without intermission, throughout the whole time.
“They have sharpened their tongues like serpents”. If still you seek to make out the man, behold a comparison. In the serpent above all beasts is there cunning and craft to hurt; for therefore does it creep. It has not even feet, so that its footsteps when it comes may be heard. In its progress it draws itself, as it were, gently along, yet not straightly. Thus then do they creep and crawl to hurt, having poison hidden even under a gentle touch. And so it follows, “the poison of asps is under their lips.” Behold, it is “under” their lips, that we may perceive one thing under their lips, another in their lips....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)