The family surname alternates between Mc and Mac. The original surname was McDonald but that obviously sounds way too Catholic, so we had to protestantise the name to MacDonald
That's a new one on me altogether. Has anyone else heard that version?
In the Church of Scotland parish registers before from 1513 to 1854 there are 31,197 McDonalds and 10,455 Macdonalds. In the Roman Catholic baptisms there are 6633 McDonalds and 3033 MacDonalds.
Or to put it another way, 31% of Roman Catholic M(a)cdonalds are spelled Mac, but only 25% of the ones in the (Protestant) Church of Scotland registers. So Mac is actually
more prevalent in the Roman Catholic baptism registers than in the Protestant ones.
That suggests to me that the idea that Mc is Catholic and Mac is Protestant does not fit the facts in Scotland.
BTW there are in total 665,693 records of McDonalds and 634,947 records of MacDonalds in Scotland's People in all time - or in other words 49% of records spell it Mac and 51% spell it Mc. There is no way that 51% of recorded people in Scotland since the records began in 1513 were Catholic.
I have heard that Mc is Irish and Mac is Scottish, but that too does not appear to fit the facts.