Is Taxation Theft?

No. As St. Thomas Aquinas argued, 

In cases of need all things are common property, so that there would seem to be no sin in taking another's property, for need has made it common. ...whatever certain people have in superabundance is due, by natural law, to the purpose of succoring the poor. ...if the need be so manifest and urgent, that it is evident that the present need must be remedied by whatever means be at hand (for instance when a person is in some imminent danger, and there is no other possible remedy), then it is lawful for a man to succor his own need by means of another's property, by taking it either openly or secretly: nor is this properly speaking theft or robbery.

I've written about this before, in my post about why I am not a libertarian, but I've been meaning to go back and edit that post. But anyways, imagine that I have labored to gain property rights over all the water in the world. Say that a man, and certainly all men but one will do, needs water, which is a natural means for the end being his survival. As he has the natural right to life, and that entails the necessity of water, then by his dire need, he has the right, by way of natural law, to take that water from me. If I deny him the water, I am essentially denying him the right to life, and the natural law does not permit me to do that. 

Now, man may be powerless to take it from you, but the state does have that power. So, the state can take it from you, and rightly so in accordance to the natural law, and give it to someone else. Likewise, with taxes.