If it’s not in the written Word, one cannot say, “Thus saith the Lord”
The Bible affirms that the era of the prophets, as with the canon of Scripture, has closed. There is no necessity for new revelations today. We already possess the sufficiency of the Bible, which declares, "Thus says the Lord." Shouldn't we consider what we have in the Bible as sufficient? Why then do people persist in either adding to or subtracting from the Word of God by proclaiming their own "prophecy," most of which fail completely? In the Bible, every word spoken by the prophets was on behalf of God and constituted the word of God. Adding to Scripture is to tread on perilous territory (Deut 4:2; Prov 30:6; Rev 22:18-29).
Regarding the legitimacy of someone claiming to be a prophet, Deuteronomy 18:22 offers guidance: "When a prophet speaketh in the name of the LORD, if the thing follow not, nor come to pass, that is the thing which the LORD hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him." God's prophets are infallible, as their prophecies always come true because they originate from God, who is never wrong. Unlike humans, God has the power to fulfill prophecies. Jeremiah states that a prophet's authenticity is confirmed when their prophecy is fulfilled, as noted in Jeremiah 28:9: "The prophet which prophesieth of peace, when the word of the prophet shall come to pass, then shall the prophet be known, that the LORD hath truly sent him." Otherwise, they are not a true prophet, but a false one.