3 “Deliver my soul, O Lord, from unrighteous lips, and from a deceitful tongue”. What is a deceitful tongue? A treacherous tongue, one that has the semblance of counsel, and the bane of real mischief. Such are those who say, And will you do this, that nobody does? Will you be the only Christian?...Some deter by dissuasion, others discourage yet more by their praise. For since such is the life that has for some time been diffused over the world, so great is the authority of Christ, that not even a pagan ventures to blame Christ. He who cannot be censured is read.
They cannot contradict Christ, they cannot contradict the Gospel, Christ cannot be censured; the deceitful tongue turns itself to praise as an hindrance. If you praise, exhort. Why do you discourage with your praise?...You turn yourself to another mode of dissuasion, that by false praise you may turn me away from true praise; nay, that by praising Christ you may keep me away from Christ, saying, What is this? Behold these men have done this: thou, perhaps, will not be able: you begin to ascend, you fall. It seems to warn you: it is the serpent, it is the deceitful tongue, it has poison. Pray against it, if you wish to ascend.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)