The aim of this chapter is to provide a description of quoniam intro ducing complement clauses, through its use in Biblical Latin, and to compare it with quod and quia in the same syntactic function. In this trio, unlike quod and quia, quoniam is not used as a complementizer before early Christian literature and Biblical translations. In Jerome’s translation, it does not fit into the quod -Old Testament vs. quia-Gospels pattern in the same way as quia (i.e. depending on the source language, namely either Hebrew or Greek), although in its use and environment it appears as a minor ‘clone’ of quia. The new syntactic func tion of quoniam introducing complement clauses is likely to result from the in creasing convergence between quia and quoniam as causal conjunctions. As a complementizer, quoniam behaves as a new and expressive variant of quia.