Qfyilyi

Rarely:

X rarely goes to Y

Frequently:

X frequently goes to Y

What would be a word in between the two? A formal word for 'sometimes' maybe?

Context:

Atheists sometimes appeal to Nietzsche's writing and use it to argue that he was against the idea of a God in general.

I need a formal word for sometimes. Frequently wouldn't be a good choice. Rarely wouldn't be either.

As I mentioned in a comment, this depends on the context. In a formal setting I might say:

X periodically goes to Y

periodically

1 From time to time; occasionally.
‘I will periodically check on your progress’

‘the students received help from a friend only periodically’

Now that you have updated the context, I would suggest:

regularly

3. usually; ordinarily.

Atheists regularly appeal to Nietzsche's writing and use it to argue that he was against the idea of a God in general.

If that seems too frequent for your needs, consider:

from time to time

Occasionally, once in a while. For example, From time to time we play bridge with the Carters.

or:

at times

sometimes; occasionally. I feel quite sad at times. At times, I wish I had never come here.

I don't know that it's any more "formal" than sometimes
(but then, I don't regard sometimes as being
particularly informal to begin with),
but a longer word for sometimes is occasionally:

Merriam-Webster:

: on occasion : now and then

//dines out occasionally

//We occasionally see deer in the field.

ODO:

At infrequent or irregular intervals; now and then.
‘ very occasionally the condition can result in death’
‘ he met her occasionally for coffee’

American Heritage Dictionary:

Now and then; from time to time.

Collins English Dictionary:

from time to time

Macmillan Dictionary:

sometimes, but not frequently or regularly
Simmer the sauce for ten minutes, stirring occasionally.
We go to the theater only very occasionally (=not often).