Civ VI introduces the concept of religious warfare. While previous incarnations of Civ use religious units like missionaries and inquisitors to spread religion, in Civ VI, apostles, inquisitors and missionaries can in addition take part in theological combat!

So rule #1: avoid expending your religious units (especially apostles with the Debater promotion). In otherwords use all but the last charge/spread.

Even missionaries, though less useful in religious battle, can be used to defend the faith after say converting a city. (Missionaries are particularly useful for Spain and India. Conquistadors escorting missionaries get +10 combat. Indian missionaries get +2 charges).

Obviously use apostles and religious units to convert cities. (But remember not to expend the unit!)

Apostles with the martyr promotion (something granted to all apostles once you construct the Mont St. Michel wonder) will produce a relic when killed. A great work that produces tourism and faith.

Even if you are going for a religious win, playing for relics is a sure bet, particularly for Poland & Kongo. Be sure to either have the belief Monastic Isolation, or use foreign apostles (martyrs of a different religion) for martyrdom.

The AI will usually convert some of your cities if you allow it. Don't fret, cos it allows you to create foreign apostles (in addition to declaring a Holy War).

In the late game you may find you need to locate that foreign capital constructing a spaceport in the fog of war. A solution: apostles!

Apostles and missionaries are great recon units. Faster than scouts, they can explore foreign lands without needing an open border agreement. So if you are on bad terms with a civ fear not. Send in an apostle. Be sure it has the martyr promotion cos chances are it will be a one-way trip.

Beware barbarians though as they will kill your religious units. (Unless it's an apostle with the Heathen Conversion promotion which will allow it to recruit barbs to escort it).

Apart from Spanish conquistadors who get a combat boost for escorting religious units, an apostle with the Chaplain promotion (medic) is useful to have on your frontlines or with an army laying siege to a city.

Position an apostle with the Heathen Conversion promotion in your cap or a city that is unhappy and about to revolt. When it does, spend a religious charge. Bam! Barbarian uprising thwarted.

*Note: This is for the base game. In R&F cities that revolt become Free States.

Should the AI be close to a religious victory (or if you're playing as the Kongo and want to convert to another religion) send apostles (or missionaries) belonging to the stronger religion into the land of the weaker religion. The AI will eventually kill those apostles shifting the balance of power.