If God is All-Powerful, He doesn’t need a Prophet to make Himself known to mankind. This implies that either He doesn’t exist, or He doesn’t want people to know Him in such an easy, automatic way. Perhaps there is value in free will, and the freedom to arrive at a knowledge of God through choice and determined effort. Perhaps that journey is part of what knowing God means.
Imagine there were a powerful king, loved by all his subjects, who wanted to choose companions for his court. He wouldn’t open his doors wide, because everyone would want to rush in. Instead, he sends out counsellors to meet the people, learn their character, test them. This is how he might find those true of heart, loyal, who love him for who he is and not what he can do for them.