Ghave has at least two +1/+1 counters on it, or there is a +1/+1 counter on another creature you control.
Devoted Druid does not have summoning sickness.
Steps
Activate Devoted Druid's first ability by tapping it, adding ({G} magic symbol) .
Activate Ghave's first ability by paying ({1} magic symbol) and removing a +1/+1 counter from a creature you control, creating a 1/1 Saproling creature token.
When the Saproling enters the battlefield, Cathars' Crusade triggers, putting a +1/+1 counter on each creature you control.
Activate Devoted Druid's second ability by putting a -1/-1 counter on it, untapping it.