In the first stanza, you've got a lamb and a child. Then, in the second stanza, the speaker throws Jesus Christ into the mix, who, in Christian theology, is often considered both a lamb and a child (and a shepherd, too, but let's not even go there). The poem is an expression of the speaker's amazement at connecting the natural and supernatural worlds, not to mention the literal and symbolic, in the figure of the lamb. The speaker's attitude represents what you could call "simple faith," a faith that doesn't have to justify itself using complicated arguments but which perhaps doesn't ask the hardest questions, either.
"The Lamb" is primarily a pastoral poem, not a religious poem. It has nothing to say about Christian faith or dogma.