Nothing is more common in this fallen world than sinners defending or otherwise covering their sin. We all desire to appear good to others and to self; and sinners desire the same while continuing to sin. In a world with laws and governments rooted in a human nature wired by the Creator to resist sinful aggressions against persons and property, keeping up good appearance is vital to mask continuous sinning.
And what is sin? Sin is any want of conformity unto, or transgression of, any law of the one and only true (Triune) God, given as a rule to the reasonable human creature.
Only one person, a man, the God-Man Jesus Christ, is perfect and sinless. All others, since the original sin of Adam or Fall of man, inherited fundamental disorder in their natures and faculties caused by that original sin, including, especially, the disordering of their ruling faculty (the will), and its misdirection toward self and away from God.
While spiritually regenerate folk, elected by God, continue to sin sporadically or on occasion after rebirth until their bodily death, the unregenerate --from the moment an active will appears in their persons in the womb sin continuously and without interruption in thought, word and deed. The unregenerate's spiritual condition is non posse non peccare, meaning, he or she is not able to not sin; and this is so regardless of what they or others feel or believe about the supposed goodness of their own intentions, motives and actions.
Sinners defend sin — in thought and word — in many and myriad ways. The heart of man, in his corrupt and fallen state, is false and deceitful above all things, and desperately wicked (Jeremiah 17: 9-10). It puts false colors upon things, plays loosely with the facts, and ends by calling evil good and good evil.
One ploy of the sinner is to deny the existence of sin altogether. Another is to obfuscate by redefining sin, contrary to what God expressly says it is. But by far the most popular tactic for defending or covering sin is by verbal deception or trick of thought, employing logical fallacies that sound plausible and even convincing to untrained ears.
A sub-discipline of philosophy, logic is the art and science of reasoning, tasked with identifying and exposing false or fallacious ways of reasoning and arguing. The various fallacies include the emotive, the ad hominem, the strawman, the red herring, slippery slope, poisoning the well, begging the question, and so on.
Easily the most popular go-to fallacy for sinners anxious to deflect notice and criticism of their sin is the ad hominem tu quoque or "you too" fallacy. This fallacy occurs when someone's argument is discredited solely based on the allegation that their past actions or words are not consistent with their present views as evinced in their argument. The intent of the fallacy user, specifically, is to discredit the opponent's argument by attacking, not the argument itself, but the opponent's behavior, past or present, as being inconsistent with the argument's conclusion. For example: Mother: You should stop smoking. It's harmful to your health. Daughter: Why should I listen to you? You started smoking when you were 16. Another example: Wilma: You cheated on your income tax. Don't you realize that's wrong? Walter: Hey, wait a minute. You cheated on your income tax last year.
Naturally, the "you too" fallacy occurs most frequently among family members and close acquaintances, when the personal history of the other person, whether real or imagined, is used to deflect criticism of self. Christ Himself was subjected to such fallacy when He returned during His earthly ministry to his hometown Nazareth, and was met by local sinners, first with the discrediting claim, "You are just the carpenter Joseph's son", and then, with deadly hurled stones. And so, as Scripture says, no man -- even the perfect God-Man Christ -- is a prophet, in this earthly life, in his own hometown.
Usage of the "you too" fallacy in argument, whether witting or unwitting-- if not called out and corrected -- leads inexorably and ultimately to a reductio ad absurdum situation, where no criticism of sin (or of anything else) is allowed because "disqualifying sin" on the part of the critic is alleged by growing legions of ever-more-sinful-and-wicked deflectors. Moreover, the nastiest variant of the "you too" fallacy, commonly employed these days by atheistic "progressives", and also a form of red herring, is psychological projection. When projecting, the sinner claims moral purity and innocence for self while preemptively and peremptorily assigning his own sinful motives and designs to his critic.
In this earthly life, by God's grace, some (but far from all) are given discernment, in varying degrees, by which to perceive the difference between good and evil, truth and error, and God's will and direction. Deceitful words, including fallacies, began in Eden with the serpent, the father of lies Satan. They will continue until the Judgment, when every knee will bend to the Creator and Judge Jesus Christ, and every tongue shall confess to God.