Section 54 of "In Memoriam" is a section where Tennyson displays some of his deepest doubts about the meaning of life and mankind's place in the universe. The entire poem was written to process Tennyson's grief over the loss of his good friend, Arthur Hallam, who died at age 22, probably of a stroke. This section was probably written about a year after Hallam's death. Various sections display greater or lesser degrees of religious faith. In this section, although the poet references God, he seems to be struggling with the idea that a God who is good could allow bad things to happen.
Thus, in the first stanza, he says that "we trust that somehow good will be the final goal of ill." The statement in itself is obviously contradictory; Tennyson's skepticism is emphasized by starting the section with "O, yet." The "yet" implies that despite what our experience tells us, despite what seems to be the case, we "trust." In other words, the trust we have seems like mere blind faith.
The second and third stanzas continue to elaborate on the perspective of one who has faith: God is the one who "makes the pile complete," that is, He decides when "one life shall be destroyed." Even among inconsequential creatures like worms and moths, death has a higher purpose, for they contribute to the food chain. If God created Nature to deal purposefully with the brainless creatures, then mankind, the crown jewel of His creation, must also not die "in vain."
Lines 13 - 16 grasp at that faith while at the same time acknowledging "we know not anything." We can only trust, based on what we see in nature, that good comes at last, just as "every winter change[s] to spring." At this point in the poem, it seems as if the poet has resigned himself to the orthodox approach of trusting that God has a good purpose for everything.
However, the last stanza throws it all away, with the speaker admitting that he is not truly able to believe what he has just argued for. The speaker calls the previous explanation of how good can come from ill a "dream." He then proclaims the depths of his inadequacy to deal with such questions:
"... but what am I?
An infant crying in the night;
An infant crying for the light,
And with no language but a cry."
The poet confesses that he is completely helpless in the face of such overwhelming questions about life and death. He feels the "night" of doubt darkening his soul, although he has been seeking the light of understanding. He does not even have words to express his anguish--an ironic statement since he has eloquently expressed himself in this section and in the rest of "In Memoriam." But all the eloquence he can muster feels to him like a mere "cry" as he is unable to truly grasp hold of faith and leave his doubts behind.