Which book discusses the concept of the washings of the New Covenant?

The book that discusses the concept of the washings of the New Covenant is Hebrews. This letter elaborates on the significance of the New Covenant in relation to the priestly duties and sacrificial system of the Old Covenant. It introduces the idea that through Christ's sacrifice, believers are cleansed and made pure, drawing a parallel to the ceremonial washings found in the Hebrew scriptures.

Hebrews specifically addresses themes of purification and how the New Covenant fulfills and surpasses the old practices. The mention of washings relates to the broader theological framework of how Jesus provides a means of true cleansing that the various ritualistic washings in the Old Testament symbolically pointed toward. This text is essential for understanding the transition from the old system of law and ritual to the new order established by Christ, making it the correct and relevant source for the concept in question.

The other choices do not primarily focus on the washings or cleansing associated with the New Covenant, as they deal with different themes and doctrinal focuses within the New Testament.