How did the Maturidi justify their belief that non-muslims must rationally find God?

One of the key positions separating the Maturidi was the position that even those who haven't been sent revelation or who haven't been exposed to Islam are responsible, because rationality can/should be used to find God and a system of ethics. My question is, what was their scriptural basis for supporting this position (whether Qur'an or Hadith or otherwise)? Any help is appreciated.