**The people could never have heard the Truth directly, but somehow they knew. I believe that "somehow" is direct from God.**
Exactly! From person to person the truth was spread. It may have started with an evangelist like St. Mark, -- It is what we, as Catholics, call Holy Triadition.
His life changed the world.
But not all these people that knew could ever have been contacted through the work of an evangelist. Some had been living in remote areas of the world for far longer than Christianity. But they too believed in Jesus and God (obviously, they didn't call him Jesus, but they knew who he was). It is proof of the power of God to transcend time and space. God spoke directly to these people. It is the only way they could have known what they did.