Moore suggested:
CHRISTIAN has come to us from Iceland, in the form of KRISTIN. The Celtic Mac was prefixed to it and then it gradually became anglicised into its present form.
KRISTIN is found in the Flateyjarbóc.
James, the seventh Earl of Derby, in one of his letters to his son said 'There be many of the CHRISTIANS in this country-that is CHRISTINS [for that is] the true name; but they have made themselves chief here.'+ It is a very common name in the Isle of Mann. In the parish of Maughold every fourth man bears it.
Compare (Scandinavian) CHRISTIAN, CHRISTIANSEN.

MACCRYSTYN [1408], CHRISTIANE [1499], ++, CHRISTIAN,§ MAC CHRISTENE [1504], MACCRISTYN, MACCHRASTENE [1511], MACCRISTIN, MACCRISTEN [1586], CHRISTIN [1610],CHRISTING [1626], CRISTEN [1632].

Maughold, Andreas, Jurby, Bride, Lezayre, Malew, Rushen, Onchan (vc), Braddan, Marown, Arbory, German, Santon, Lonan (c), elsewhere (u).

* Four Mast., Vol. II., p. 694.
+ Manx Society, Vol. III., P. 49.
++ This is given in the 'Statute Law Book' as 1419, which cannot be correct.
§ Christian is not the usual form till the 17th century.

see www.manxnotebook.com/manxnb/v10p053.htm