The stories I've read say that you're supposed to build several small temples and harvest wine as well as gold to do this but this doesn't work. All my settlers seem to do is store gold and send wine to my temples for magical spells but the magical spells I have don't promote my soldiers. I'm playing as the Romans, I'm on the second level of the game and my soldiers are being slaughtered by the enemy because they're only standard infantry, they're on 61% strength. How do I promote them?

Each faction has a favored valuable recourse (I think Roman is gold). Load up on it, keep it in a storage bin on your turf, and your soldiers will get stronger. You can also purchase your upgrades but it's been a while since I played it so I can remember exactly how.

that actually helped, thank you

Not exactly.
You promote soldiers with alcoholic drinks sent to small temples. For Romans it's wine, beer for Egyptians and rice wine for Asians.
On the soldiers tab you can see DIVINE PROMOTION %. Once enough alcohol reaches small temples and you see 100% you can promote 1 type of soldier by clicking on its icon. It takes longer for every next promotion... Once all are promoted to sergeants, you can start promoting to officers same way.

Gold affects OFFENSIVE %, which affects the strength of your soldiers when fighting outside their country. Egyptians only get half effect from gold, but they are also the only faction which can use jewels for fighting strength and they receive double effect from them.

Attached image in case my expanation is not clear

Man, it really has been a while since I played. :P Good thing he didn't give me credit for a right answer. XD