"Render to all what is due them: tax to whom tax is due; custom to whom custom; fear to whom fear; honor to whom honor." (Rom. 13:7). "He who steals must steal no longer; but rather he must labor, performing with his own hands what is good, so that he will have something to share with one who has need." (Eph. 4:28).
From Al Capone to Willie Nelson, you don't cheat on your tax return. People who cheat, according to CNN Money, are the same people more likely to keep the wrong change given to them by a cashier, to ask a friend to pretend to be a former boss for a reference check, to lie about their income to qualify for government aid, to wear an outfit once and return it, to file false insurance claims, to keep money they see someone drop on the floor, and to lie about finding something inappropriate in their food just to get a free meal. It’s theft -- and Christians do not steal. We're to be benefactors, having something to share with the needy, not beneficiaries skilled in the ways of deception. Our testimony is tied to our work ethic, which is God's remedy against stealing. Ironically, cheating cheats the cheater. First, it "cheats" his conscience of its clarity. No one wakes up and decides to swindle -- it's a learning curve. It's also a galvanizing one. Stealing becomes borrowing, borrowing slides into ownership, ownership hardens into entitlement. The end result is the law of the jungle and a seared conscience -- an impossible state in which to serve God (2 Tim. 1:3).
Second, it "defrauds" his perception of social equality. The cheater feels more entitled than the other party in the exchange -- "My need is greater than theirs." "They’re richer than me, they won’t miss it." "They don’t deserve this as they are crooks themselves." The perceived imbalance is based on a perceived, or even a real, injustice -- always in the favor of the one who thinks more highly of himself than others. It starts with the inequity of doing what is right in our own eyes, usually with the principle that if it's okay to lie to a liar, then it's okay to cheat another cheater -- whether that cheater be the government who cheats its citizens by printing money it doesn't have in the treasury, or a con man who steals from elderly citizens.
Third, cheating ruins or "devalues" our testimony as Christians before others. If the lost aren't saved through the cleverness of our preaching (1 Cor. 1:17), neither will they be through the cleverness of our dealings -- even if it's legal. Put yourself in the other party's shoes. Would he feel defrauded? If so, then take the high ground and take the perceived loss and keep your light shining. What makes for a good Robin Hood movie usually makes for a lousy testimony.
Don't cheat -- especially on your taxes. You lose far more of yourself than you'd ever gain -- hardens your sensitivities to doing right, builds an inequitable discrimination toward others, and shrinks a testimony of where your true treasure lies.