Is it possible to define your own short cuts in LibreOffice Writer?

Can you define a key sequence that when typed will automatically be replaced by a specific word. For example can I define a sequence say “cx” which will replaced by a word say “christianity”. Whenever I type cx it will automatically be replaced by the word christianity.

Yes this can be done!

Writer > tools > autocorrect options

on top of the opening window select the language in which you want to use the short cut

tab Replace > enter “cx” and delete if there is something right to “cx”

tab With > enter “christianity” > click new

Make sure you have a blank after the “cx” and “christianty” string

Make sure the the language setting for text matches the language you selected for the short cut.
Tools > language> and then choose what you need.