If my cleric takes the Ritual Caster feat (choosing the cleric list), can I copy all known ritual spells into my ritual book to cast them at any time?

Question

I would like to have access to my class’s ritual spells all the time without having them prepared. I might be about to justify taking the Ritual Caster feat if I can make it do what I want it to.

If I take the Ritual Caster feat (and choose my class's spell list for the feat), can I copy all the ritual spells that I have prepared from my class into my ritual book, and thereby cast them as rituals even when I haven't prepared them?
Or do I have to find each of those spells on a scroll somewhere to copy from?

For instance, as a level 3 cleric, there are 7 spells in my list of 1st- and 2nd-level spells (from all different sources) with the ritual tag that I can prepare – but I have other things I’d rather prepare. So if I take the Ritual Caster (Cleric) feat, can I just copy down all of those spells into my ritual book from memory?

My goal is to no longer have to prepare those spells, and just be able to cast them (only as rituals) from my ritual book.

Answer 1

You must copy the spells from another written source.

The Ritual Caster feat (PHB, p. 169) details what is required for copying a spell into your ritual book:

If you come across a spell in written form, such as a magical spell scroll or a wizard's spellbook, you might be able to add it to your ritual book. The spell must be on the spell list for the class you chose, the spell's level can be no higher than half your level (rounded up), and it must have the ritual tag. The process of copying the spell into your ritual book takes 2 hours per level of the spell, and costs 50 gp per level. The cost represents material components you expend as you experiment with the spell to master it, as well as the fine inks you need to record it.

No provision is made for copying the spell into the book from memory.

Using the rules for Scribing a Spell Scroll in Xanathar’s Guide to Everything (p. 133), you could get the rest of your rituals without having to rely on the DM to let you find them:

With time and patience, a spellcaster can transfer a spell to a scroll, creating a spell scroll.

However, the rules detailed in this section are optional rules, and can only be used with the DM’s permission, so you’re still relying on your DM somewhat to allow you to fill your ritual book.