The Ahlul-Fatrah are those whom the call of the messengers did not reach.
"What is apparent is that what is correct about this question - which is, 'Do the mushrikun (those who associate partners with Allah) have an excuse due to their being people whom the call of the Messengers did not reach or not?' is that they have excuse to it in this world and that Allah will test them on the Day of Resurrection with a fire which He will order them to enter. So whoever enters it will enter Paradise and they are those who would have affirmed the truth of the Messengers had come to them in this world but those who refuse to enter the fire then they will be punished in it and they are those who would reject the Messengers if they had come to them in this world since Allah knows what the people would have done had the Messengers come to them."
What is established from Allah's Messenger and it is a text directly relevant to the matter about which there is no disagreement. So there is no room for disagreement to continue along with it. And from the proofs that the people of Al-Fatrah will be tested is the saying of the Prophet:
"Four will make a plea on the Day of Resurrection: A deaf man who could not hear, an insane man, a senile man and those who died in a time when there was no Messenger. So as for the deaf man, he will say, 'O my Lord, Islam came and I did not hear anything'. As for the insane he will say, 'Islam came while the children threw dung at me'. As for the senile he will say, 'Islam came and I could not comprehend anything'. As for the one who died in the time when there is no Messenger, he will say, 'O my Lord your Messenger did not come to me'. So their agreement will be taken that they will obey Him then He will send a Messenger to them ordering them to enter the fire. By Him in Whose hand my souls is, if they would enter it, it would be cool and safe for them."
From On the Issue of Takfeer