Depending on the type of the tax, the income it is being forced upon, as well as where the money are being used, it may or may not be theft.
If I can barely afford to eat (let alone pay rent), and can't put some money aside, then I consider taxation a theft, UNLESS they provide me with housing, food and medicine. Then, tax away.
I understand that governments need money to pay salaries and a huge amount of other stuff, but that's 100% what I feel.