I will raise up a prophet like you from among their fellow Israelites. I will tell that prophet what to say, and he will tell the people everything I command him…You may wonder, 'How will we know whether the prophecy is from the LORD or not?' If the prophet predicts something in the LORD'S name and it does not happen, the LORD did not give the message. That prophet has spoken on his own and need not be feared. (Deut. 18:18-22 NLT)
Moses didn't live forever. When he was about to leave this life, he spoke of other(s) who would follow him. These people would speak for God as well. They would hear what God had to say and pass the message along to the rest of us. There was a term we learned to use to describe these people. The term is "Prophet."
A Prophet, or "truth-sayer," would bring additional revelation from God. This was different from a teacher or rabbi who would simply teach us what God had said in the past. Prophets had something new to say because God had something new to do. The question, of course, is obvious. How do we know if someone is truly a prophet sent by God and not a fake sent to deceive us. The answer (or part of it, anyway) is simple: Miracles.
People today seem to gravitate to one extreme or another when it comes to miracles. On one extreme, the scientific-minded try to explain away miracles as being impossible because they simply do not match reality as we experience it on a day to day basis. On the other extreme, religious-minded will often see miracles as normal everyday occurrences (at least for those living in biblical times). Neither is true.
Miracles DID happen and they were NOT normal occurrences. That's WHY their accounts are captured in the pages of the Bible. Miracles were used to get everyone's attention...even now. They were used to show us power far beyond what we normally experience. They were used to let us know that God had something to say and that we should stop what we're doing and listen. God used miracles to confirm certain people as His Prophets...His voice. Then, we could trust the words the prophets had to say.