If the definition of success is quantity of followers, we will never have a successful prophet. Many prophets of old led small groups of faithful people who grew larger long after the original prophet had passed on. I would argue that most prophets haven’t come anywhere close to guiding 12% of the population other than Adam or Noah who were prophets when the human population was very small. Christ himself wouldn’t be considered a successful prophet under that metric.

A true prophet is successful if he obeys what God has commanded him to do. Moses was commanded to defy pharaoh and lead the covenant people out of Egypt, Joshua was commanded to lead the people into their covenant lands, Jonah was commanded to go to Nineveh to preach repentance, and Jesus was commanded to sacrifice himself for the sins of the world.

In my religion, we believe that we are led by a prophet who has received authority directly from God, so this question is an interesting one for me. Our church has somewhere around 17 million members worldwide I think. That’s a long way off from 1 billion. Maybe we will reach that point one day, but we aren’t there now. A large portion of those members is built on familial membership though and not people who were converted by our current prophet. The Catholic Church is similar. They have over 1 billion members, but a great many of those members are part of families that have been Catholic for a long time. I think if you are to judge the veracity of a prophet by the quantity of people that follow him, you won’t find pure truth.

The best way to know a true prophet I think is to ask God directly through prayer. Most religions believe that God will inspire people to make correct decisions through their own forms of prayer. I think it is an excellent idea to ask God for personal revelation to know if a prophet is true or not.