Contemporary usage has modified "belief in all religions" to refer more to an acceptance of the legitimacy of all religions. The OED elaborates that an omnist believes "in a single transcendent purpose or cause uniting all things or people".
It seems to be a position rather than a religion but, as usual, it seems to mean slightly different things to different people sometimes. It doesn't seem a term non-believers need use much but perhaps one worth knowing.
Another is orthopraxy...
In the study of religion, orthopraxy is correct conduct, both ethical and liturgical, as opposed to faith or grace etc. This contrasts with orthodoxy, which emphasizes correct belief, and ritualism, the use of rituals...
1. correctness or orthodoxy of action or practice.
Then there's occasionalism....
The doctrine that God is the sole causal actor and that all events are merely occasions on which God brings about what are normally thought of as their effects.
occasionalism - belief that interactions between mind and body are intermediated by God.
I think religionists invent lots of concepts that really aren't worth another word.