Prophets speak what only God knows and desires to communicate. It is not uncommon. Jesus said, "'When He, the Spirit of truth comes . . . He will disclose to you what is to come'" (Jn 16:13). It usually is about the future. Isaiah said, "'Let them bring forth and declare to us what is going to take place'" (Isa 41:22). He said to , "'Cry loudly, do not hold back'" (58:1). Jeremiah said, "'The prophet who has a dream may relate his dream , but let him who has My word speak My word in truth'" (Jer 23:28). Jesus presented himself as "'the faithful and true Witness'" (Rev 3:14). To be a valid prophetic word it must be from God and sent by him. "'The prophet who prophesies of peace, when the word of the prophet comes to pass, then that prophet will be known as one whom the Lord has truly sent'" (Jer 28:9). Moses instructed, "'You may say in your heart, 'How will we know the word which the Lord has not spoken?' When a prophet speaks in the name of the Lord, if the thing does not come about or come true, that is the thing which the Lord has not spoken'" (Dt 18:21-22). Jeremiah cautioned to not listen because "'they speak a vision of their own imagination, not from the mouth of the Lord'" (Jer 23:16).
There are hundreds of Old Testament prophecies many of which have been fulfilled in the New Testament and others which still must come to pass. For instance Isaiah prophesied, "'The Lord Himself will give you a sign: Behold, a virgin will be with child and bear a son, and she will call His name Immanuel'" (Isa 7:14). This was fulfilled by the birth of Jesus Christ. He reiterated, "'For a child will be born to us, a son will be given to us; and the government will rest on His shoulders; and His name will be called Wonderful Counselor, Mighty God, Eternal Father, Prince of Peace'" (9:6). Was the meaning clear at that time or was it a mystery? Daniel said to the king, "'There is a God in heaven who reveals mysteries, and He has made known to King Nebuchadnezzar what will take place in the latter days'" (Da 2:28). But the king didn't know the meaning so Daniel responded, "'I will read the inscription to the king and make the interpretation to him'" (5:17). In Paul's case he said "by revelation there was made known to me the mystery" (Eph 3:3). He understood because "the righteousness of God is revealed from faith to faith" (Ro 1:17). Without God's inspiration man tries to understand with his intellect by questioning. But "one who doubts is like . . . a double-minded man, unstable in all his ways" (Ja 1:6-8). Therefore you must believe in prophecy from the heart in faith.