Today it would be absurd to think that there is someone alive who had known someone who had known Christ. But when we look at the patriarchs from Genesis 1 to Genesis 11, this could type of thing could have been possible even until Jacob was a small child.
Shem, the son of Noah, lived until the time of Abraham, Isaac and Jacob. His grandfather, Lamech, who he could have known for almost 100 years, could have known Adam for 56 years.
Putting that into perspective, that means that if that kind of longevity had continued, our grandfathers could have known someone who was alive at the time of Christ and known him.
A lot of times we think of God as being almost omnipresent in those time of the patriarchs, but He appeared to them only a few times in all that time. Those men lived by faith the same as we live by faith today.