Accusations of heresy did not arise in a vacuum during the Middle Ages. Polemicists and inquisitors had their own agendas, often involving lay or ecclesiastical politics. Struggles between lay and clerical leaders could descend into accusations of failing in faith. Even political rebels could be accused of heresy. Heresy and treason became equated by the thirteenth century, opening the way for stronger measures against dissenters. Polemical works used the accusation of an error to put an opponent on the defensive. Opponents replied in kind. The importance of the idea of heresy in the West is underlined by comparison with the Islamic world. Muslims accused political foes of apostasy, entirely abandoning the faith, not erring in its interpretation and teaching.