If Islam is a complete religion, why did it not reach to everyone with the same vigour. The Non-Arabs during Prophet's time, the people living in forests, the nomads who are born and die without getting to hear Prophet Muhammad(pbuh)'s name and yet they are in his Ummah. Why did the Nubuwat not reach them and how exactly will they be reckoned on the Day of Judgement?

If they did not know of Islam nor had the ability to find it then they would be considered Mustad’af. They will be tested in the next life before being judged.