The English language is complex, as it has changed a lot over the course of… about 1,5k years. Having only 26 letters, it has about 46 sounds, and as far as the relationship between spelling and pronunciation is concerned, it is quite irregular. Though I’d argue that when one has been learning it for years, or using it consciously, you can spot the patterns and regularities in the language, however irregular it may be.
Correct - this is the pronunciation. The ‘t’ in ‘christmas’ is still there (in spelling), even though it is not pronounced. Likewise 'christianity' may be pronounced with ‘sh’ or ‘t’ - there are variants which defy standard grammar rules.
As for xian vs xtian - we know that ‘christ’ is fictitious, not some guy by the name of Chris.
Just my two linguistic cents