Wordsworth was something of an idealist. No, not the kind that wants to save the whales and solve global hunger tomorrow. Granted, we don’t think he had anything against the whales…that we know of. But by "idealist" we mean someone who takes the philosophical position that the external world doesn’t give us a true picture of reality, but that the things in the world contain an inner "essence" of truth. This essence always remains the same, even when external appearances change. The British Romantics, like Wordsworth and Coleridge, were heavily influenced by German Idealist philosophers like Immanuel Kant. In "Mutability," Wordsworth compares the process of change to a melting frost: the appearance may change, but the thing itself is still there, frost or not.
Wordsworth believes only in transcendental truths accessed by the imagination and intuition, and not in truths demonstrated empirically or through language.
The speaker’s absolute trust in the processes of nature is akin to a religious faith. However, this faith carries with it no comfort of the afterlife.