Faith and religion do not contribute to fame or any victory or end game condition in HUMANKIND. (Well, not directly).

The One True Faith achievement requires you convert 7 other empires. So you need to have at least 8 competitors.

I suggest a smaller world size: Normal (usually for 6 players) instead of Large. Or minimum land percentage.

If you choose Large, choose continents. Pick 8. Otherwise max Elevation for more mountains. And more lakes and rivers.

Highly recommended: Peaceful mode.

To make your religion successful and ultimately dominant, you need followers.
The more followers, the more dominant your religion will be.

So for example in the early game if you play as an Agrarian culture like the Harappans, Celts or Garamantes, you can grow your cities using their affinity action.

The first threshold is 10 pop, where you get to choose between Shamanism and Polytheism.

At 25, you will get to pick your religion and first tenet.

You want to focus on food and growth. And build as many holy sites and faith producing emblematic districts as possible.

So instead of expanding and trading in the early game, continue this isolationist "food + faith" strategy until you have reached the 4th and final religious threshold of 165 pop and tenet.

In HUMANKIND building a unit costs a population.

If your goal is to grow eventually reaching 165 population (for the 4th tier tenet) you want to avoid building units.

As a rule of thumb, in the early game only build units when your city population is capped and growth is negative.

In HUMANKIND the AI very aggressively pursue religion. Religion spreads via roads, harbours and trade. So play an early isolationist game avoiding trade.

Keep an eye on your faith in the Religion screen.

In the later game, build units in cities of a different faith.

I only recommend the 4th tier tenet Donate Generously. Unlike all the others, it allows you to build an extra two holy sites, each of which produce 20 faith.

Because you are playing an aggressive religious game, it is essential you pick one of these cultures in the Ancient Era. Early game culture suggestions are far more important than late games ones.
Harappans, Bantu
Aksumites, Celts, Garamantes, Goths, Mauryans, Maya
Aztecs, English, Franks, Teutons, Umayyads
Edo Japanese, Haudenosaunee, Maasai, Ottomans, Spanish
Mexicans, Russians
Brazilians, Indians, Nigerians, Turks

Try to build as many cultural wonders that double as holy sites. Especially in the early game.

In the Ancient Era there is Stonehenge and the Temple of Artemis. Try to get at least one of these.

Failing which try one of the Classical wonders: Statue of Zeus and Church of Our Lady Mary of Zion.
Medieval: Angkor Wat, Great Mosque of Djenne, Notre Dame, Tōdai-Ji.
Later: St. Basil's Cathedral and Christ the Redeemer.

Pick civics and event choices that tend towards the Tradition ideology instead of Progress; World over Homeland.

Again this is to boost faith and food output.

After you pick your first religious tenet the civic Religious Tolerance will unlock. Pick Religious Hostility. Notably this will grant you a grievance against empires of other religions. A grievance that you can use to war with them.

Clearly empires following other religions are going to be your targets for war.

Conversely empires that follow your religion are good candidates to trade with and to form alliances with.

When you do go to war target territories with holy sites and wonders that act as holy sites (like Stonehenge).

If you demanded that empire convert to your religion as a grievance, then when you win the war, that empire will have to adopt your religion as their state religion, as conditions of their forced surrender.

Whenever you choose an expansionist culture, you will have the ability to annex foreign territories every 10 turns.

For example when playing as the Teutons or Russians, make sure you grab as many territories as possible in those eras. Obviously you will target empires territories of different religions. Prioritise those with holy sites and wonders that generate faith.

This is where Peaceful mode helps.

(Of course if there is an army garrisoning the Administrative Centre then you are out of luck!)

Once a foreign territory has been annexed you may want to wait a few turns until it has converted to your religion first before attaching it to a city.

Take a look at the Religion screen. Often it will tell you how many turns you have to wait before it converts.

Note that it will have to be defended by your military until then.