Roman 1:20 For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse, because although they knew God, they did not glorify him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened.
God has placed the knowledge of him in our hearts, but he gives us free choice to walk with him or not, and that walk is based on the knowledge that he has given you. In that case, their very life and lifestyle indicates a knowledge of God.
This is why we are to share our faith with others. its also extremely important that we raise our children to be obedient to God to read the bible and pray because they will not be spared. The children in conquering nations were not saved. Children have some knowledge of Good and Bad, Right or Wrong. They will be judged as adults, now I was stunned when I first read it but its true, as you go through those conquering nations you never hear of them being saved. there was one instance in where they saved some and the woman because that particular tribe of Israel was almost no more and they need more people to secure the tribe, I hope that makes sense.
Jesus really covered everything. There is no excuse for anyone to say they do not know the lord. Especially in this generation, but I know Christians don't know the Lord either, so that passage above would apply to them as well.