Instructions
Eat and drink food and beverages that increase your Purity alignment. Go to the pub in town and speak with the owner to see what he has for sale. As you highlight each item, the in-game description tells you how it will affect your alignment. For example, eating meat increases your Corruption while eating tofu increases your Purity.
Decrease the amounts you charge your tenants for renting your property. Walk up to each property you own and interact with it --- just like when you bought it in the first place. This brings up a menu with which you can increase or decrease what you charge for rent. Lower it to the absolute minimum to get the fastest increase to Purity.
Marry someone. If you've been taking the time to develop a relationship with any of the characters, you can eventually marry her. Purchase a ring from the Jeweler and flirt with your love interest until you have the option to propose. You must own a house to get married. Once you are married, your Purity increases.
Make donations to the Temple of Light. If you aligned yourself with Evil early on in the game, you likely destroyed the temple. If you didn't, you can interact with the pedestal next to the Abbot to make donations. Every donation you make increases your Purity.