Is someone who has an income that he does not pay tax on sinning?
I cannot answer every possible issue related to your question. There may be some gray area here. I do not know what country you live in, which would also affect my answer. In the United States there are gray areas, such as if a person mows their neighbor’s lawn and is paid $10 or if we sell a $2 item in a garage sale it may not be necessary to file a tax form for that miniscule transaction. Fourteen year olds doing babysitting do not need to pay taxes. However, as a general rule, in most countries, if one has a “job” which includes wages for that job, then they must pay taxes on the wages earned in that job. To willfully break the law is a sin according to the Bible. Matthew 22:15-22 makes Jesus’ thinking on this question clear. He says that, despite any moral or ethical concerns, we should “pay taxes to Caesar.” Paul agrees with Jesus on this, not surprisingly. He says in Romans 13:1-7 that we should submit to governing authorities, “not only because of possible punishment but also because of conscience.” Paul does not state any exceptions.
I do not know the context of your question. You might be asking this question so that you can prove to a friend that he/she ought to pay taxes, or this may be a concern about your own situation. I know that there will be reasonable exceptions. A person living in Syria, making income in a rebel-held territory, might not even know who to pay his/her taxes to. A Jewish person living in Nazi Germany, making a tiny amount of money to stay alive, who would be killed if he/she were to pay taxes might be an exception. The problem is that everyone thinks that they are an exception, so I say that, not given any context to your question, then the answer is clear cut. Yes, it is a sin for a Christian to have earned income and to not pay taxes on that income.