CARDINAL

major, as in a cardinal sin

serving as an essential component - "cardinal rule"

SYN - central, fundamental, key

"It wasn't like you could dismiss the resurrection and still hold onto a Christian faith - accepting the ressurection is a cardinal part of what it means to be a Christian."

"It was the cardinal idea of an immaterial element to human's which is where his opponents had decided to attack his thesis."

"It was a cardinal mistake from which he would never be able to recover."

"He had commited a cardinal sin and it would be a long road to complete forgiveness for the young man."