Wheat, heat, tomato, onions all existed before a sandwich was invented. That doesn't make the meal people had before that a sandwich. The sandwich is a particular way of bringing these together.
God, followers, holy men all existed long before Islam was invented. Islam was a particular way these were brought together. Followers of this new religion are called muslim. Just like shops selling the sandwiches are called sandwich shops
Word Christian means follower of Christ, Jews are people of Juda tribe or sons of Juda, Islam means submitting yourself to the will of God and who does that is called muslim, they're not called Muhammadis or Muhammadans.