Securing a Religious Victory inCivilization 6requires a lot of Missionaries and Apostles to spread your faith far and wide. Of course, other empires will try to do the same thing. An opponent dedicated to religious dominance can be frustrating, since a large group of Missionaries proselytizing in your empire is tough to deal with.
If you’re constantly fighting to keep your cities from being converted, you’ll probably need to change your approach to dealing with enemy Missionaries. Here are some ways to keep them at bay so they don’t lead your people astray.

Make A Diplomatic Request
When one of your cities is converted by a rival’s religious unit, you canspend Diplomatic Favorto ask them to stop sending Missionaries. If they accept, they get the Diplomatic Favor that you spent andwill suffer the penalties of breaking a promiseif they attempt to convert your cities again in the next thirty turns. That gives you time to recruit Missionaries of your own and convert citizens back.
If the opponent refuses your request or breaks their promise, you’ll gain Grievances against them, letting you take more drastic action without losing standing in the eyes of the world as long as you respond quickly.

Theological Combat
The best defense against Missionaries is toengage them in Theological Combatwith Missionaries of your own - or better, Apostles. Defeating a religious unit this way not only removes them from the map, but it alsoreduces the influence of their religion in nearby cities.
Try to engage enemy Missionaries while you’re standing on a Holy Site. It provides a significant bonus in Theological Combat, and heals weakened religious units each turn.

It’s important toalways have at least one Guru nearbywhen you’re fighting a war of words with rival preachers. Gurus canheal religious units, helping them survive longer against large groups. This is also important for keeping your hard-earned Apostles alive.
Convert Your Opponent Instead
A city can only create Missionaries of its dominant religion - that is, the religion followed by the most citizens living there. If you convert your rival’s cities to your own religion, they won’t be able to keep recruiting Missionaries of their faith.
Try converting City-States on the border first, to increase religious pressure.

Apart from all-out war, this is the most expensive but most permanent way to stop waves of Missionaries. There’s also a hefty sum ofEra Scorein it for you if you convert their Holy City.
Declare A Holy War
If you don’t have enough Faith to recruit Missionaries, Apostles, and Gurus in sufficient numbers, your only other recourse may be war. TheHoly WarCasus Belli is valid against any player who has converted one of your cities, but any Casus Belli will work just as well if it’s later in the game and you’d prefer a different one.
While you’re at war with a player, your military units candestroy their Missionariesusing thePurge Hereticaction on the same tile. This earns you some Faith and is themost efficient way of removing Missionaries from the map.
Of course, if you’re going to commit to a war, it’s best if you canweaken or even conquer your opponent.Pillaging their Holy Sites will set back their plans considerably, and taking them will be an even bigger blow. No Shrines means no Missionaries!
attempt to time your attack with a World Congress resolution targeting your religion with a buff or theirs with a debuff. It will make combat much easier.