The act of believing implies an inner assent, but it is neither limited to a blind acceptance of certain arguments nor to an empty repetition, without practical consequences, of a set of concepts that one claims to defend. This article, allied with the philosophy of language (Austin) and the philosophy of science (Lakatos) will aim, on the one hand, to present a philosophical–theological reading of believing and, on the other hand, an understanding of the implications of the assent of faith regarding the Nicean Creed for Christian identity. The research briefly presented here will refer to the firm core of Christianity, as well as the arguments that aid its defence and understanding vis-à-vis believers and non-believers.