The Barry wrote: “. . . there's no ‘meaning’ behind the wearing of a kilt pin.”

True enough. But one is free to attach a meaning to it, and a symbol that speaks of Christianity is (for a Christian) a highly appropriate choice.
Aside from those already suggested, one could also use a pin showing a dove, representing the Holy Spirit.
Then there is the shamrock, which not only symbolises Ireland, but was used (as legend has it) by St Patrick to explain the mystery of the Trinity to the High King at Tara. (Not solely a suggestion for those of Irish ancestry.)
There are many possibilities.
Regards,
Mike