The message of "To India My Native Land" is a simple one, yet one built on complex poetic devises and one embedded in history and revealed through allusion. To begin to understand the message of the poem, first understand the history driving the dominant metaphor that begins, "let me dive into the depths of time."
In its glory days before British Empire and colonization, India had a role of prosperity and power among pre European empire civilizations. The antiquities and artifacts of India's "days of glory past" attest to the truth that India was reverenced and metaphorically "worshipped as a deity" among other civilizations.
With British colonization, India was subjugated and divested of glory as the conditions of the world were permanently altered by imperialism. India is described as having its "eagle pinion" chained down as an eagle might be bound to immobilize its wings and subdue it by imprisoning its primary ("pinion") feathers in chains.
The poet expresses the message of the poem in the metaphor of diving into time past--the pre-colonization days of India's glory--to scavenge and bring out some "few small fragments of these wrecks sublime" of memories of what once was India's glory. Like a love sonnet, the poet would immortalize India's past grandeur with the words of his poem. He requests in the sonnet's concluding rhyming couplet that his reward ("guerdon") for so immortalizing India be that some reader(s) might give "One kind wish for" India.
And let the guerdon of my labour be,
My fallen country! One kind wish for thee!
To summarize, the message is that this poem, written in the early 1800s, is designed to remind people of India's fallen position, fallen from past wealth, power, and glory and to promise India that he shall find in the ocean of past time the fragments of past wonders so that the world might give a kind wish (instead of a destructive wish) to the once great civilization of India.