In the context of Pauline theology, what is the significance of the term 'Mysterion' (μυστήριον) as it relates to God's plan of salvation revealed in the New Testament?

A It refers exclusively to the sacrament of baptism. B It denotes a secret ritual practiced only by initiated Christians. C It describes a previously hidden truth now revealed through Christ and the Gospel. D It signifies the unknowable attributes of God beyond human comprehension.