Roman2: 12 For as many as have sinned without law shall also perish without law: and as many as have sinned in the law shall be judged by the law; 13 (For not the hearers of the law are just before God, but the doers of the law shall be justified. 14 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: 15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;) 16 In the day when God shall judge the secrets of men by Jesus Christ according to my gospel.
God showed Himself and saved men prior to Christ, God declared Noah righteous by His grace.. , God revealed Himself to Abraham and His off spring
If God had so ordained a man to be saved, He would make that man aware of Himself and His coming mercy . The OT saints were saved looking forward to the cross (Gods mercy) but they never knew Christ or Christianity.
What about the Indians in America prior to 1492? They had no knowledge of Judeo-Christian teaching (unless you believe the Mormons), but presumably there were some altruists who did good deeds just to do them.