is there a item / permanent spell / summon to make priests stealthy ?

[LintMan]

One thing you can do is to recruit/summon a stealthy unit (ie: a indy scout, dusk elder, or harvester of sorrows), and make that unit your prophet.

Then you'll have a stealth prophet that spreads dominion without needing to preach. (And if it's a Harvester of Sorrows, it'll also spread disease in you enemy's provinces as well.)