If ritual is repetition outside of what can be justified through direct experience, can ideas and opinions, as well as words and actions, be ritualistic? Would a belief in God - given the axiomatic impossibility to verify the entity directly - classify as ritual?
Till the experience of clear seeing and knowing, realizing, what ever deed by body, speech and mind is good to considered as a habitual ritual, householder Ilya Grushevskiy and interested.
Ways of belief, expecting certain happiness and release, to the extend of not knowing, not clear seeing, are rituals.
In regard of abounding the fetter of ritual toward an aspiration headed to Nibbana it requires the knowledge of the path through realization. Relaying on this, it's also again practiced as ritual (habitual) but no more destined toward anything else as well as raw rituals not headed toward the aim have been abounded.
The same, of course, would count also for the path into the Brahma-realms, if realized once.
To possible understand the underlying point as well: believing is an action (mental), a practice, as well, and the foremost in becoming also signs and physical deeds, rituals. Usually the word ritual is used for practice that would not really cause the desired effect (i.e. actually: mind is one, sign, physical another). Sometimes ritual refers also to holding on certain actions by signs and deeds although the mind might not be directed at it's purpose, perceived as habitual only outwardly.
(Note that this gift of Dhamma is not dedicated for trade, exchange, stacks or entertainment but as a means to make merits toward release from this wheel)
I get the impression (e.g. from this answer) that a "belief" might (instead) be classified as a "view".
The word you're asking about might be sīlabbata -- using a quick search of Google I saw several discussions of that, but didn't notice references to suttas where the term is clearly defined.
The PTS dictionary says,
bbata [=vata2] good works and ceremonial observances Dh 271; A i.225; S iv.118; Ud 71; Sn 231, etc.; sīlavata the same Sn 212, 782, 790, 797, 803, 899; It 79
Given that the word includes sīla I assume that, like the precepts, it implies some element of behaviour and not only of view.
I think the two (i.e. "belief" and "practice") might be equated by some Christian sects, e.g. "the only thing you have to 'do' is 'believe'" (i.e. the Christian sola fide doctrine) -- I guess that might seem like a ritualistic belief.
There may be something analogous in Buddhism too, e.g. this answer or perhaps even just taking refuge.
Is ritual always a practice, or can it be a belief too?
Silabbata paramasa is a ritual backed by wrong belife.
If a ritual is a repetition outside of what can be justified through direct experience, can ideas and opinions, as well as words and actions, be ritualistic?
Yes. As it is a practice not rooted in reality.
Would a belief in God, given the axiomatic impossibility to verify the entity directly - classify as a ritual?
Yes. Any practice expecting liberation will be.
Following is a good writeup:
SILABBATA PARAMASA is generally translated into meaning the adherence to wrongful rites, rituals and ceremonies. Believing that a wrong practice is a right practice is called Silabbataparamasa, which is believing, maintaining, or supporting a wrong belief in the practice. According to the teaching of the Buddha, apart from the Eightfold Noble Path, all other practices are wrong practices and taking them as right practices amount to wrong belief in the practice.
Everything that appears at the six doors of senses constitute the Five Aggregates of Grasping, namely, rupa and nama, the Truth of Suffering. Meditating on rupa and nama is practising the Path by which the Four Noble Truths will be understood. Believing in and practising any other method which keeps aside the magga Path and which does not lead to understanding the Four Noble Truths, is wrong belief in the practice (silabbata paramasa ditthi).
There are people who are preaching that "It is not necessary to practise meditation nor to observe the precepts (sila)." That is to say, they are saying it is sufficient to simply listen to sermons and learn by heart the nature of rupa and nama." It will be necessary to consider whether such views amount to silabbata paramasa. In the opinion of some, such preachments amount to teaching wrong view in practice as this method excludes the three disciplines of:
Although a person at a lesser level of insight may participate in Wrong Practice and even be unaware of such, a Sotapanna, a person of the first level of The Four Stages of Sainthood, and those above, being well-established in the knowledge of the Right Practice are not liable to hold the wrong view of silabbata paramasas. (1)
SILABBATA PARAMASA DITTHI PRESENTED BY: the Wanderling
I think you can easily distinguish between the two. Beliefs are orthodoxy i.e. excepted views, doctrines, or teachings. Ritual is orthopraxy or correct conduct, liturgy, or action. What ultimately distinguishes ritual from belief is that the former is expressed in some context whereas the latter fundamentally structures how a person views the world. Saying "I believe in God, the Father, the Lord, the Giver of Life" is to make a ritualistic declaration of an orthodox belief. Belief is God, on the other hand, is to formally order your universe according to that view. Obviously, both are contrived when compared with direct experience.