In New Testament times the Greek verb pisteuein, to believe, normally takes the dative. In the New Testament the verb sometimes appears with the preposition eis (into, to) instead. Without a proper analysis this construction has generally been taken to bear the same meaning as the construction with the dative. But there are good reasons to question that. The construction with eis does not occur before the New Testament, and up to about 200 CE, at least, it appears only in Christian texts. Furthermore it is used almost exclusively when the preposition is followed by Christ or the name of Christ or the like. That indicates that this is a linguistic innovation, consciously made to express a specific Christian conception or experience of faith. Possibly it should be associated with the idea, found in for example Paul, of Christian life being led "in Christ" and with early Christian notions of baptism. The aim of the project is to study the meaning of this linguistic innovation and its origin and development in the early church up to about 200 CE. The study includes analyses of the separate occurrences of the expression in early Christian literature, analyses of tendencies and lines of development in the use of it and reflections on its origin. The outcome of the analyses can entail a partly new understanding of the conception of faith in early Christianity, of its distinctive character, origin and development. It can also throw a new light on a number of texts from this period.