The classical `decision problem’ asks whether a given formula of first-order logic is satisfiable. In this work we consider an extension of this problem to regular first-order \emph{theories}, i.e. (infinite) regular sets of formulae. Building on the beautiful classification of syntactic classes as decidable or undecidable for the classical decision problem, we show that some classes (the EPR and Gurevich classes) which are decidable in the classical setting are undecidable for regular theories; on the other hand for each we show a subclass which remains decidable in our setting, leaving a complete classification as a challenge for future work. Finally, we observe that our problem generalises prior work on verification of uninterpreted programs, and give a semantic class of existential formulae for which the problem is decidable.