This would be a GM call, but I would say the next generation and from then on would be a D8 spirit instead of the D 10 mainly for one reason.
1. The second generation,
unless they walk with their god directly could never match the spirit of their fathers in faith. Their fathers walked with their god. If their children do not then it would reduce. Even though you spend the points on a generation's worth of spirit, having that high of a spirit score is supernatural.
In order to keep the D 10 for the second generation I would say the god does not have to spend the full miracle amount, but make his presence felt (ie. a ghostly visage every holy season, small miracles such as rain when needed and so forth, just listen to your follower

That would translate to at least 1 belief extra on top of the miracle costs per year to keep the followers believing (upkeep).
I hope this answer helps.
Richard Leon
Owner