Byzantium is in a tough spot here since most crussades march through her and is surrounded by Catholics and Muslims.
I found out what better worked for me as Byzantium was an early drive to raise my churches decently, build up one major priest recruiting ground with a cathedral (so that your priests come out already as Bishops) which will attract (already olny because you do not have much competition) the big Orthodox Theological guild houses.
Reinforcing the major enemy priest entry points with Icon paint shops helps also quite some as it raises 100% your conversion bonus.
Do not forget that no matter how many priests you have in one region, a certain % of your religious level will always fall when an enemy priest marches in. Its like a quaranteed % which once reached 80% to 75% becomes more and more tough to breach.
You getting swamped? Retaliate and you will see enemy priests running home to try to stop your own conversion spree on his lands.
Finally..with the much improved assasin figures you can deal with traspassers too.
Religious assaults along with some spies and assassins will pave your way to many regional take over without having to declare war on your neighbor. Just hit them till they revolt and then come in as a peace keeping force
