Cult of the Lamb

First, I'm going to say I do think this achievement is cumulative. At the time when I unlocked this, I had 11 Disciples simultaneously, and one of my previous Disciples passed away from old age. Since this update is fairly recent at the time of writing, I'm not sure 100%, but from my own experience and another comment here, it seems like it may be the case.

Second, something to keep in mind is that you do need to sacrifice a Level 10 follower to the Silk Cradle door to unlock it as part of the Relics of the Old Faith update. So, if you're returning to the game with both updates to complete like I am, just something minor to be aware of.

After that, once I got a follower to Level 10 the second time, they requested I perform the ritual called "Ritual of Discipleship". I don't think this was available previously; but, the nice thing is after you unlock it for the first time, it's located on the first row of Rituals and you can perform it immediately after a follower reaches Level 10. There is no need to wait for them to request you to do it.

Now, how about gaining some loyalty? There are plenty of ways:

1) Mating. As a part of this update, you can have followers mate. You want to pick two that have a high chance of making a baby. There's an achievement for hatching 5 eggs anyways; however, the benefit of this approach is the option to "Nurture Child", which can be done once a day, fully levels up the kid. Eventually, they become an adult, and they can be around Level 5-7, depending on the traits. So, it's a nice head start.
2) Rituals. There are few rituals you can perform to raise all of your followers' Loyalty. The first is "Alms for the Poor". You spend about 25 gold to increase their loyalty. The best one is "Ascend Follower Ritual". This will also increase all of your followers Loyalty, and by a significant amount depending on the follower you choose.
3) Dailys. Daily things like Confession, Sermon, Bribing, and Inspiration. Confession can of course only be used once daily, and on a single follower, but it is a huge boost. Bribe and Inspire are actions you can choose within the "Interact" wheel.
4) Items. There are certain necklaces that can help. The first is "Loyalty Necklace". This necklace makes it so the follower never Dissents, and they gain Loyalty 25% faster. Something to note about Disciples in general, is that they are not immortal. However, once a follower does become a Disciple, they live for quite a long time. So, just in case my theory about this achievement being cumulative is incorrect, some other necklaces to keep an eye out for are the "Skull Necklace", which lets a follower live for twice as long, and the other is the "Golden Skull Necklace", which makes a follower immortal. You can also gift followers things. While out on Crusades, be on the lookout for "Small Gifts" and "Large Gifts". There is no limit to how many gifts you can give a follower per day. The only limit is how many you have..
5) Quests. If a follower asks you to do a quest, consider doing it as you'll gain a good chunk of Loyalty with them if you do.
6) Cooking. The "Magnificent Mixed Meal" has a 100% chance to increase follower Loyalty. It's not a lot, but every bit helps. There's also the "Splendid Vegetable Feast", but that only has a 50% chance to increase follower Loyalty.
7. Monch. Ok, technically this is a way, but in all of my Crusades since starting this game up again, I ran into him once. He's an NPC that can be found in a room marked as "?". So, do not rely on him, I'm really just letting you know he exists.

I think that about covers it. Good luck!

Once you have a level 10 follower, they will ask you to make them a disciple and unlick the "ritual of discipleship" ritual, only level 10 follewers can become disciples.