God's messenger to his people
A biblical prophet was first a messenger. God told Moses that he would raise up a prophet and put his words in that person's mouth, so the prophet would speak what God commanded (Deuteronomy 18:18). That is why prophets often introduced their messages with words such as, "Thus saith the LORD". They were not simply religious advisers offering personal opinions.
The Hebrew word often translated prophet is nabi, which carries the idea of someone called or appointed to speak. God could call people from very different backgrounds. Amos was a herdsman and a grower of sycomore fruit before God sent him to speak to Israel (Amos 7:14-15). Jeremiah felt young and unprepared, yet God sent him where he was told to go and gave him words to speak (Jeremiah 1:7).
I will raise them up a Prophet from among their brethren, like unto thee, and will put my words in his mouth; and he shall speak unto them all that I shall command him.
More than predicting the future
Prophets did sometimes speak about what was ahead. Isaiah spoke of a coming child whose rule would bring peace (Isaiah 9:6), and Daniel was given visions concerning future kingdoms (Daniel 2:44). These messages showed that history is not outside God's care.
Still, prediction was not the centre of their ministry. Isaiah confronted Judah's empty religious activity and called for justice, mercy, and care for the vulnerable (Isaiah 1:16-17). Nathan challenged King David after his sin against Bathsheba and Uriah (2 Samuel 12:7-9). A prophet told the truth when it was costly, especially when powerful people preferred silence.
Wash you, make you clean; put away the evil of your doings from before mine eyes; cease to do evil; Learn to do well; seek judgment, relieve the oppressed, judge the fatherless, plead for the widow.
How were prophets tested?
The Bible does not ask people to accept every spiritual claim without question. A prophet's message had to agree with faithful worship of the true God. Even a sign or prediction was not enough if the speaker urged Israel to follow other gods (Deuteronomy 13:1-3).
God also gave a practical test for predictions spoken in his name. If a claimed prophecy did not come to pass, Israel was not to treat that speaker as having spoken for God (Deuteronomy 18:21-22). Jesus similarly warned about false prophets and said their character and results would reveal them, like a tree known by its fruit (Matthew 7:15-16).
Jesus and the prophetic message
Christians believe the prophets prepare the way for Jesus. After his resurrection, Jesus explained to his disciples that the Law of Moses, the Prophets, and the Psalms pointed towards him and needed to be fulfilled (Luke 24:44). Peter later connected Jesus with God's promise to raise up a prophet like Moses (Acts 3:22).
That doesn't make the prophets distant figures with nothing to say now. Their books keep inviting readers to trust God, turn from wrongdoing, defend those who are mistreated, and hold onto hope. Read one prophet slowly, perhaps Jonah, Amos, or Isaiah, and notice both the honesty of God's warnings and the steadiness of his mercy.
And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.