Also not easy. You get to wear your biases on your sleeves, depending on your audience. (So, is this history for Christians or is is history about Christians for the academy?) But commenting theologically on particular history is pretty tough. It tends either towards the banal (the quasi-baptismal sprinkling of pieties on top of broad brush-strokes) or to the apocalyptic-demagogic (myopic tub-thumping from the vantage point of the hobby horse). An example of my failure to stay atop the two stools, or to find another good launching point, can be found amid the NTI papers.
However, when the historian in question can write really well, and I mean really well, then it can be pulled off. Think of accessible scholarly works… Stephen Neill, The History of Christian Missions (Penguin, 1964) is a great example of this, as is Henry Chadwick’s The Early Church. Back in them days they know how to write. Polite but determined swashbuckling. Due to my deplorable ignorance of the field I hesitate to generalize and there could be a hundred fabulous examples out there that I’ve never heard of, but recently only one book has really grabbed me. Jonathan Fuller’s Cross Currents: the Story of the Muslim and Christian Encounter in the Philippines (OMF, 2005) has much smaller ambitions than either of those classic works but is a delight. Passionate, scrupulous and vivid. [Sadly, it doesn't look available except in the Philippines at the moment :-( ]