Thanks for asking. The Quran and Prophet Muhammad (pbuh) has clearly told us Islam has always been the religion revealed to mankind from Prophet Adam (pbuh), i.e. the very first human. There have been over 114,000 messengers (which includes prophets - we can leave the semantics of this for another time) of Allah sent to different peoples throughout the history of human kind. Allah has never let humans be without right and wrong to choose from. Prophet Muhammad (pbuh) was the final messenger and he brought the final message to the whole of mankind (not just a group) which would forever be available and unchangeable.
Allah's message to different people has eventually always been corrupted by man's ego massaged by the devil. All except the final message of course, which the devil is working hard on changing interpretations but obviously the original source will always remain unchanged/pure.
To conclude, Allah has never left mankind on their own. It's always been man's choice to abandon Allah. Those that choose to follow Him always had/have the opportunity to do so and always will.