The GotQuestions.org article Are there prophets in the church today? asks the following rhetorical question (in bold):
Are there true prophets today? If the purpose of a prophet was to reveal truth from God, why would we need prophets if we have the completed revelation from God in the Bible? If prophets were the “foundation” of the early church, are we still building the “foundation” today? Can God give someone a message to deliver to someone else? Absolutely! Does God reveal truth to someone in a supernatural way and enable that person to deliver that message to others? Absolutely! But is this the biblical gift of prophecy? No.
Notice that the question relies on the premise that "we have the completed revelation from God in the Bible".
What is the basis for the claim that "we have the completed revelation from God in the Bible"?
Note: I'm not asking for the basis for the belief in a closed canon. Although that topic is related, notice that it is possible to believe in both a closed canon and the continuation of non-canonical (divine) revelations.
- How do continuationists respond to the 'closed canon' argument against the continuation of special revelations and sign gifts?
- How do non-LDS believers in the modern restoration (or continuation) of the offices of apostle and prophet rebut GotQuestions.org's objections?
- How do Latter-day Saints rebut Don Stewart's article "Are There Still Apostles Today?"?
Appendix - Clarifying terminology
Ironically, I find the following quote from this GotQuestions.org article quite handy:
The canon of Scripture refers to all the books in the Christian Bible and Hebrew Scriptures that together constitute the complete and divinely inspired Word of God. Only the books of the canon are considered authoritative in matters of faith and practice. The idea of a closed canon is that the Bible is complete; no more books are being added to it. God is not appending His Word.
Emphasis mine. In other words, a closed canon, to my understanding, means that God has no further plans to providentially intervene in human history to add new chapters or books to the Bible. That's it. It doesn't mean that God cannot reveal new things outside of the Bible, although those new revelations will not be appended to the Bible (to remain consistent with the premise of a closed canon). For example, someone might be taken up to the third heaven and receive revelations from God that no-one else knows, and yet the canon would still remain closed because those revelations would not be appended to the canon. There is no contradiction.
By non-canonical (divine) revelations I mean revelations from God that are not officially recorded in the Biblical canon. I recommend reading this answer to the question What was the purpose of the Old Testament prophecies whose contents were not recorded for posterity? to better understand this point.
UPDATE: I also recommend reading this answer to the question Is there a contradiction between Hebrews 1:1-3 and Acts 21:8-11?