In spite of all we say about grace—in spite of all we know about salvation by grace through faith and not of works—we still find warnings in the Bible about things which, if not followed, will keep us from ever seeing God. There is no misunderstanding this. Salvation is by grace through faith. It is not a matter of works. (Not that we aren’t expected to work, but the work is not saving.) Then we have this statement from Hebrews.
Follow peace with all men, and holiness, without which no man shall see the Lord.
The absence of holiness means we don’t see God. That means we aren’t there to see him. And it is an instruction to us that we are to “follow holiness”. That means there is something for us to do, a way in which we are to walk. Perhaps a way to understand this is: We have to be made holy. But having been made holy, we must follow holiness. Our problem is understanding just what that means.