According to "Christianity: The First Three Thousand Years" by Diarmaid MacCulloch, The Sultan's army that was marching towards Constantinople was majority Christian mercenaries. Can someone provide more context on how that happened?

Quote from the book (Chapter 14)

Now there were only months left before the Ottomans closed in on Constantinople. The Emperor Constantine had at best eight thousand soldiers to defend it against Sultan Mehmet II’s besieging army of more than sixty thousand, backed by many more miscellaneous supporters.^(47) To call it a struggle of Muslims against Christians would ignore the fact that the majority of those fighting for the Sultan were Christian mercenaries.

Cited source in the footnote:

47: R. Crowley, Constantinople: The Last Great Siege, 1453 (London, 2nd edn, 2006), 102–3; Crowley provides a careful and vivid account of these events.