Final answer:
The term 'sacramentum' was used for baptism as it represented initiation into Christianity, analogous to purificatory rites in Mystery religions, but with a unique transformative effect on the believer's life.
Step-by-step explanation:
The word sacramentum was first used to describe the Christian ritual of baptism because it signified initiation into the Christian faith. Much like the Mystery religions that often featured purification rites, Christian baptism served as a powerful symbol and act through which individuals were incorporated into the Christian community. The heart of this sacrament is the belief in purging original sin, marking the transformative entry into a new spiritual life.
When considering the significance of baptism, it is important to note that it was not just seen as a one-time purification, but as a permanent change in the individual's spiritual status, distinguishing it from other purificatory rites. Additionally, the mystery and sacredness of Christian sacraments, especially baptism, began to echo the secrecy and profundity of the ancient Eleusinian Mysteries, as hinted at by church writings. With the spread of Christianity and the backing of influential figures like Constantine, these rituals became foundational to community life, encapsulating the struggle of good versus evil and marking significant life stages.