Rapid scientific advances and technological changes in the nineteenth century led Victorians to feel anxious about their society, their historical views, and how they saw themselves. This century saw a very rapid scientific leap forward, as society reaped the rewards of the investments in factories and industrial technology begun in the eighteenth century.

Further, the invention of the railroad shortened distances between places, and the beginning of modern medicine began to give humans control over their bodies and a reasonable chance to fight disease in a rational way. Most radically, even before Darwin published On the Origin of Species,scientists were beginning to challenge accepted and seemingly settled knowledge about the origin of the world and humankind. The authority of the Bible as an account of God's creation of the earth, including when and how it happened, increasingly seemed to run counter to scientists' geological and biological findings. People became anxious, as it began to appear as if humans had no special destiny and might simply be another species of animal, perhaps here for just a moment and, like the dinosaurs, doomed to extinction.

In "In Memoriam," a requiem on the death of his good friend Arthur Henry Hallam, Tennyson reflects on some of these Victorian anxieties. While he yearns to understand his friend's death within the context of a Christian narrative in which God's love redeems the world, he wonders if that is possible in light of scientific depictions of nature as characterized by a state of pitiless warfare. As he puts it, contrasting religion and nature,

[We] Who trusted God was love indeed
And love Creation's final law—
Tho' Nature, red in tooth and claw
With ravine, shriek'd against his creed.

Is it true, Tennyson wonders, that nature's violence "red [bloody] in tooth and claw" is the essential reality, rather than the Christian message?

He expresses his doubts with a cry from his heart by saying "we know not anything" and calls himself

An infant crying in the night

and

An infant crying for the light.

At this point in the poem, he is distressed and lost, feeling as helpless as an infant to understand the nature of the universe, reflecting the feelings of many people of his era.

Directly addressing the doubts science brings, he applauds the idea of facing them squarely rather than denying them, stating,

There lives more faith in honest doubt,
Believe me, than in half the creeds.

Tennyson finally comes to a resolution which reflects the flip side of the Victorian age's anxiety: his era's faith in progress. He envisions science and Christianity coming together, both calling forth—one through evidence, the other through faith—a new and better age. The world may be evolving, but it is evolving in the right direction, as he notes in the final stanza:

That God, which ever lives and loves,
One God, one law, one element,
And one far-off divine event,
To which the whole creation moves.

Tennyson's poem tries to honestly articulate Victorian doubts and fears.

I would also suggest looking at what eNotes has to say about this poem: the analysis is very good and touches on the questions you raise.