Why did the vowel in "Christ" become long in moving from Old English to Middle English?

Long and short vowels were written the same way in Old English (macrons, the lines marking long vowels, are a modern convention). Given the current pronunciation of Christ, I’d guess it had a long and not a short vowel in Old English. But I’m not sure. If it did have a long vowel, the reason might be because Latin Chrīstus apparently had a long vowel.

The Ormulum (early Middle English) uses the spelling Crist (where the non-doubled s indicates a long i) alongside Crisstene and Crisstenndom.

If the word really did have a short vowel in Old English, French influence could possibly be a reason for the pronunciation with a long vowel in Middle English. Monosyllabic words ending in /st/ taken from French, like coast, toast, feast, fairly often have long vowels for some reason.

I looked at the Oxford English Dictionary entry for Christ, and it says the length of the vowel in Old English is "uncertain".

It isn't a case of homorganic lengthening of the type found in child: that lengthening process was an early sound change that only regularly applied before consonant clusters ending in a voiced consonant, not ones like /st/ ending in a voiceless consonant.