It is difficult to imagine Christianity without grace.
While opinions may differ about what is non-negotiable in Christianity—the things a person must believe, value, or practice to remain within the boundaries of the tradition—grace seems about as essential as anything.
A shorthand for grace I heard as a teenager was “getting something you don’t deserve.” That feels like a harsh way to put it! That wording works well if the best metaphor for our relationship with God is one of, say, benevolent but strict parent to fundamentally naughty child.