I think you were right both times Barry. The speaker in the poem does not exactly believe, but he projects onto the bird an incomprehensible full-throated belief whose source is unknown to him. Much was written on another thread of how the bird "had chosen thus to fling his soul/ Upon the growing gloom." I called this suicidal. One reader described this as a metaphorical funeral pyre. What I had in mind was "Empedocles on Etna," the subject of a major poem by Arnold, whose "Dover Beach" is discussed elsewhere because of its fascinating "darkling" connection to both Keats and Hardy. In Arnold's important preface of 1853 he rejected "Empedocles" because of its pessimism (which it shared with nearly all his poetry). He insisted, from his new classical perspective, that no great poetry could be written out of situations "in which the suffering finds no vent in action; in which a continuous state of mental distress is prologed, unrelieved by incident, hope, or resistance; in which there is everything to be endured, nothing to be done."

You might say that Hardy picked up the torch of pessimism that Arnold was casting aside, but learned from Arnold by also dramatizing the alternative--or is the illusion?--of hope, as represented by the thrush.