It is certainly not a good practice to allow an employee to use the church credit card to make purchases for others who use church as their banker. It might violate your credit card agreement (you would have to read the really fine print), but it is probably not illegal. It could be illegal if the church has a sales tax exemption and the other organization does not so that the “church purchase” allows the other organization to avoid paying sales tax on the items. You ought to ask this employee directly why he is doing it and, if you are uncomfortable (as you probably should be), tell him to stop or take away his credit card.