What does it mean in Adam all die?

What is the meaning of Whereas in Adam all men died so in Christ shall he, et cetera? 

It means just what is written. In Christ was introduced a new element into humanity—spiritual life—and by partaking of it, all men may become part of him, and through him, of God. Adam was the representative of human life—Christ is the representative of spiritual life, and humanity embraces all who have ever lived or ever will live on your earth.

Were not people saved before Christ came into the world?

They were saved because Christ represented them as part of humanity.

How did those who lived before he came know anything of him?

Christ was always, and you are thinking of his manifestation. They accepted the light that was given them. You must not forget that this Divine love always existed. He came in a natural form, but he said himself—Blessed are those who, not having seen, yet have accepted him.

—Insight in Spirit