what would they have thought was the most effective thing to do with their money?

One of the comments from MR is:

Probably hire a bunch of mercenaries and engage in forced conversion to Christianity. For the pacifists, perhaps funding missionaries might have seemed more acceptable. What better investment can there be for an effective altruist than eternal salvation of lost souls who will otherwise face an eternity of damnation?

which basically does sound about right, given the values of the time.