No, taxation is not theft.
And He said unto them, "Render therefore unto Caesar the things which be Caesar's, and unto God the things which be God's". -- Luke 20:25
It is becoming more and more fashionable in libertarian and Tea Party circles to proclaim that taxation is theft. It is not, and we need to stop saying that. It is un-Biblical, is is poorly considered public policy, and it makes us look so extreme that we are dismissed no matter how good our arguments are on other matters.
First and most importantly for Christians, saying that "taxation is theft" is a direct contradiction of our Lord Jesus Christ. Civil government throughout the Old Testament collected taxes, including to build the Temple, with the blessing of God. Given that God commanded "thou shalt not steal" and God is incapable of sin, taxation cannot be theft. We are not more holy than God, and it leaves our political opponents room to mock us by pointing out that Jesus takes the opposite position than we do.
Secondly, it is bad public policy. Government does need to do certain things. The most obvious Biblical reason is to bear the sword against criminals, which requires a criminal justice system complete with lawyers, police and judges and the support staff for all of them. We also need to defend our nation against foreign aggression, which requires a military. We need to collect taxes to pay for those things. One could argue (as I do) that government at all levels is far too big and needs to be slashed, but even if we shrink government to an ideal libertarian level we will need to collect taxes to run that.
Finally, repeating the "taxation is theft" mantra is politically self-destructive. In addition to the hypocrisy angle for Christians who defy the Lord Jesus, people know that there basic things that government needs to do and that those things need to be funded somehow. So when we say "taxation is theft," we look like extremists and people will dismiss us as fringe. Worse yet, they will laugh at us - and being a laughingstock is political death. Nobody wants to be led by a clown.
A more nuanced view is that taxation is not theft, but taxation can be theft. Corrupt politicians who enrich themselves with money forcibly confiscated from the people, or use that wealth to bribe their way to even more power, may be guilty of theft though taxation. It is not as catchy of a slogan and does not fit on a bumper sticker, but "taxation can be theft" followed by an explanation is a position that can be defended and explained. Let's do the hard work of making that argument, instead of posting cheap slogans on social media.