non agression pact, so nothing hostile
no attacking or casting rituals on someones lands, and no stealthy preachers. preaching within your own lands and building temples and the like is all fine. as for moving stealthy units, you're probably safest letting the person know you're going to do it so they don't get too ticked off if something goes wrong. and that way they can let you know which provinces to avoid (if they're patrolling or have high PD or something).