A flat rate income tax is not prohibited biblically, just so long as it does not reach the 10% level (I Sam. 8:14, 17). However, it is unlikely politically to remain biblically restrained. Voters will seek to tax higher-income residents at a higher rate: a denial of the rule of law (Exo. 12:49). A sales tax is an inherently flat tax. Everyone pays the same.
- Gary North, Taxation in the Bible, at http://www.garynorth.com/public/2315.cfm.