Kongo is a really interesting civ, but isn't it a bit too good being next to them? I mean, you can have a free boost to your religion just spawning behind them! This could change the balance in the game, I hope this will not happens!
How can you hope this will not happen when this is exactly what will happen? You just described one of the outcomes that will be regularly happening with Kongo (closest civ to them that founds religion is going to have easier job spreading it)

I fail to see what's wrong with it, such dynamic interactions are fun and I prefer "reverse balance" of "everything is powerful" to flat balance "all abilities barely impact game".