In his splendid biography of Thomas Huxley, Adrian Desmond lists the three articles of faith underpinning Huxley's agnosticism (Huxley invented the term): "physical causation, the uniform order of nature, and an objective world."
In other words: There is a world outside of the human mind; nothing happens in that world that does not have a cause which is itself part of the world; those causes are orderly and can be known with an ever-increasing degree of verisimilitude.
Huxley knew these statements could not be proved. His belief in the unbreakable orderliness of nature was no less a matter of faith than the theist's belief in divine interventions, even if deductions from the assumption of order "are always verified by experience." His task then (as Desmond writes) was to discredit those who claimed miraculous deviations.
More than a century later we still await any verifiable claim of deviation.
Of course, claims of divine interventions, rewards, punishments, answered prayers, and miracles are almost universal, and those who believe in them are not likely to be persuaded by a lack of evidence that might convince a skeptic. And so the agnostic and the theist reside in their separate faith positions without much hope that the one will convince the other.
Huxley the agnostic had in his favor only a close shave with Ockham's razor -- he dismissed the apparently superfluous. When he died, the Catholic Review snarkily opined: "He is no longer an Agnostic; he knows now that the Christian revelation is true."