The answer is "Openbaring".
In Dutch, "Afsluiter" means "concluder" or "closer," and "het Nieuwe Testament" (often abbreviated as "N.T.") refers to the New Testament of the Bible. "Openbaring" is the Dutch word for "Revelation," the final book in the New Testament.
Therefore, "Openbaring" fits the clue of being a 5-letter word that refers to the conclusion of the New Testament.