As I tried to understand it, and when I try to explain it, it often strikes me that Christianity is a bit complicated. The essence cannot be explained in a sentence, not even in a paragraph.
At one time in my life I thought that this complexity was because Christianity was not true, therefore it lacked the concise precision of science and was forced to create a model of reality which I found difficult to hold in my head all at once.
Now I recognize that Christianity is complicated because of The Fall, because of Original Sin.
The 'original plan' for humans was a simple one, but the complexities introduced by the fact of free will of angles (i.e. the fall of angels, Satan and the demons) and the fall of man (shortly described in Genesis, but its consequences everywhere) then made the Christian plan of salvation considerably more difficult to summarize.
Christianity as we have it is God's Plan B - therefore to understand it requires that we know Plan A, how it went wrong, and how it was set right.
That, I think, is the main reason why Christian salvation is somewhat slow to be explained.