I think the question lies in whether tsuba were made specifically to represent a Christian theme or did particular patterns become popular with Christianity for reasons such as convenience, preference etc. There does not seem to be a Christian tsuba tradition of tosogu even though certain tsuba categories are regarded to have a tendency towards Christian motifs.
Charles Sanders Pierce wrote
"a 'symbol' is a sign whose special
significance or fitness to represent just what it
does represent lies in nothing but the very fact
of there being a habit, disposition, or other
effective general rule that it will be so
interpreted"
In others words, if it walks like a duck, sounds like a duck it can only be one thing depending on what you think. These tsuba as they stand can represent Christianity if you want them to and I think there is nothing wrong with that. The question is were they being made to cater for a Christian believers market or just a popular non referential, non symbolic style that became a choice of an (enlighten) minority?