Use 'axiomatic' for statements so clear that they don't need additional evidence or justification. To experts, it's axiomatic that the Earth orbits the Sun.
Pro Tip 2/3
Common Agreement
'Axiomatic' implies wide acceptance or common agreement on a fact or idea. It's axiomatic among teachers that reading enhances vocabulary.
Pro Tip 3/3
Implicit Trust
Using 'axiomatic' suggests you trust the listener to accept the statement without doubt. In customer service, it's axiomatic that the customer is always right.