what i did was earn as many souls as possible (which means once you find all 3 stones for the cross, do not auto absolve) each of the 27 shadows you encounter, absolve them, and use all your souls on the holy path. As you progress upgrade you holy path, and you will need to get to Holy Level 7 before all upgrades are available. I had around 9,000 souls left over as i hit level 7 and was able to purchase the final upgrade. The upgrades range from 1,500 to 6,000 souls, and it helps to absolve the gluttons and demons as they usually give u between 100-390 holy points per absolve. You will also get the *Soul Reaper* achievement for collecting 60,000 souls along the way.
Hope this helps, i'm more than happy to add anything if I've missed something so please message or comment me b4 giving a negative vote :)