It is nature that provides Wordsworth with those "Intimations of Immortality" mentioned in the title. When a child grows up, the adult has lost not only the proverbial innocence of childhood, but a sense of immortality as well. In the eighth stanza, Wordsworth addresses the child he used to be as a philosopher and a prophet, who had access to truths which he has since forgotten. However, he prophesies that a child who is at one with nature will inevitably become a mere mortal:

Full soon thy Soul shall have her earthly freight,

And custom lie upon thee with a weight,

Heavy as frost, and deep almost as life!

The later stanzas of Wordsworth's “Immortality” Ode demonstrate the intellectual maturity of the speaker's mind, how he's now stirred to deep thoughts by the contemplation of nature. When he was a child, the speaker derived feelings of immortality from his many happy interactions with nature. And though memories of these bygone days can still provide the catalyst for a surge of childlike joy in the adult speaker, they have been superseded by a more mature joy, inspired by a felt consciousness of his mortality derived from observing nature.

There is no real tension here between earthly and heavenly existence. The tension is rather between how the speaker used to see the natural world as a child and how he sees it as an adult. When he was a boy, the speaker's interactions with nature would give him an elevated feeling of immortality. But now, as an adult gazing upon the natural world, he experiences a profound intellectual joy from an awareness of his mortality.

This newfound perspective on things supersedes his worldview from childhood, while at the same time making room for memories of younger days that still bring him considerable joy. In that sense, the tension is resolved. The speaker's outlook on nature may have changed, and he may have come to a greater degree of philosophical maturity than he ever developed in his youth, but he can still treasure those wonderful memories of his formative years when his soul was infused with intimations of immortality.

The conflict in this poem doesn't seem to me to be about the conflict between earthly and heavenly existence. I think the poem makes it fairly clear that the conflict is based on the way in which we are able to perceive the full glories of nature as a child, and that we gradually lose this ability as we grow older. This is what the final two stanzas of this poem refer to. The poem ends as the speaker of this Ode recognises and accepts the waning of his faculties in regards to being aware of the joys of nature and being open to learning of them, but with an insistence that he will still do his best to rejoice in the beauty of nature:

What though the radiance which was once so bright

Be now for ever taken from my sight,

Though nothing can bring back the hour

Of splendour in the grass, of glory in the flower;

We will grieve not, rather find

Strength in what remains behind...

Even though the same kind of visceral and intimate connection with nature, it is suggested, is now no longer possible, the speaker resolutely determines not to "grieve" in this loss. Instead, he will focus on what "remains behind" in terms of his ability to meditate upon the glories of nature. The poem ends then not in a tone of pessimism but in one of optimism as the speaker celebrates the strength of the human heart and recognises "man's mortality" in how aging necessarily involves a loss of perception of nature and of its beauty.

Thanks to the human heart by which we live,

Thanks to its tenderness, its joys, and fears,

To methe meanest flower that blows can give

Thoughts that do often lie to deep for tears.

Age necessarily involves diminished perception, but the speaker determines to use what faculties remain to him to continually rejoice in nature.