Taxation is not theft because taxation is a legally established process for funding a society, and theft is the unlawful taking of something without permission.

Taxation is not theft because taxation is a legally established process for funding a society, and theft is the unlawful taking of something without permission. You do not need to personally consent to or agree with all laws for them to apply to you. You are also free to make taxation a point of civil disobedience, if you feel that strongly that it is immoral. You are free to start a movement to eliminate all taxation, and if you succeed in having it become the law of the land, then taxation will no longer exist, of course, but it would still not be theft under any circumstances.