The simple answer to these questions are: Tax evasion is a sin, but not a mortal sin…as far as my research takes me. I say this, because I can find no authorative statement from the Vatican to confirm that tax evasion is not a mortal sin. And the precedence set from such sources as the Baltimore Catechism (which claims that stealing can sometimes be a venial sin, if the amount is small).
Now some Catholics might wonder why this would matter. A sin is a sin, and venial or mortal…it’s something that we should avoid. And this is correct. But the question pops up every now and then (especially during the filing season) and the object here is simply to proclaim the truth. Or at least, the as much of the truth as we can discover.
Again, we can find no official documents concerning the gravity of tax evasion. It is clearly defined as a sin in the Catechism under article 2409 - “The following are also morally illicit… tax evasion”. It is also mentioned in article 2240 – “Submission to authority and co-responsibility for the common good make it morally obligatory to pay taxes, to exercise the right to vote, and to defend one's country: Pay to all of them their dues, taxes to whom taxes are due, revenue to whom revenue is due, respect to whom respect is due, honor to whom honor is due.”
But the severity of the sin isn’t defined (at least, as far as my research takes me), so we must rely on our understanding of the sin itself, Biblical passages, and common sense. First, we know that it is a form of stealing. In America’s case, it is stealing from the common good.
Secondly, we know that stealing can be a mortal sin in some cases, and a venial sin in other cases. The line between these isn’t (and couldn’t be) clearly defined. But there must be some distinction between a child sneaking into the cookie jar, a clerk sneaking into the petty cash funds, and a mugger holding up a victim.
The question of the sin’s severity rests in the amount that is taken, and the thief’s circumstances. For example, someone taking food in order to survive (so long as it doesn’t hinder another person’s ability to survive) isn’t a sin at all. But if the person were to take much more than they needed, it does become a sin. If the person were to take enough to have an impact on the other person’s life, it would become a mortal sin.
In the case of the government (the common good), the amount one individual contributes in tax money is a drop in the bucket. The government has plenty. But that drop in the government’s bucket might be huge to the individual filing taxes. In this, there’s no evidence that tax evasion is a mortal sin.
The Biblical passage people point to most concerning taxes is Mt:22:21 - “…Render therefore to Caesar the things that are Caesar's; and to God, the things that are God's.”
Remember that the question they were asking was, “Is it OK to pay tribute to such an evil empire.” This is the same argument used by Christian tax evaders today. “Why should I give money to a government who spends it on abortion, etc.?”
Jesus’ answer is that money changing is entirely a man made thing. And that their petty concerns are just that…petty. Think about the context of the question the scribes were asking. They were trying to set a trap for Jesus, and they got more than they bargained for.
Jesus turned it all around to show that not only could people pay taxes, but that the leaders should quit bickering over superficial matters. They wanted to approach him about something serious, and Jesus publicly made them look like children.
And from all this, the only conclusion one could draw, is that tax evasion is not a mortal sin. But because it is a sin, it should be a voided. Further more, the US Government doesn’t take it so lightly.