"The Lamb" represents Blake's vision of heaven as an orderly place firmly under God's authority. The speaker of the poem, who we discover in the second stanza is a child, is addressing a lamb and telling the creature that it was created by God. He refers to this creator god as the Lamb, meaning Jesus. The lamb is thus not simply created by God, but is in the image of God.

The lamb is also a symbol of the human child. The child speaker says that he, too, is made in the image of God, because both are called lambs, just as Jesus is:

I a child & thou a lamb,

We are called by his name.

As other posters have previously noted, one of the overriding themes of Blake's "The Lamb" is the wonder and the simplicity of creation. The speaker addresses a little lamb, asking it whether it knows who made it and gifted it all its glorious qualities, such as its soft wool and its sweet voice which cheers all who hear it. The speaker goes on to explain to the unknowing lamb that all of this, marvelous though it might seem, was created by God, and the tone of the poem expresses great wonder at God's works.

In his 1789 poem, William Blake reflects on the traditional conception of God as Creator. In the first stanza, the speaker first asks the lamb if it knows who made it, expanding the question into a description of the lamb's wonderful qualities: white, soft wool and a pleasant voice that uplifts the valleys in which it grazes.

In the second stanza, the speaker informs the lamb that "He" who created it has imbued it with some of His same qualities: meekness and mildness. He alludes to Jesus, who is traditionally thought to have been born into humanity as an embodiment of God. "We are called by his name" calls to mind the idea that Christian followers consider themselves sheep in the flock that follows their divine shepherd. At the same time, it evokes the idea of a sacrificial animal, just as Jesus gave his earthly life for the redemption of humanity.

The poem is deceptively complex and belies the idea of a simple theme statement. Because Jesus is both shepherd and sheep, creator and embodiment, it could be said that a theme in the poem is the mystery of faith and the unknowability of the Creator of both mankind and the natural world. Before the final couplet, the speaker identifies himself as a child, separate from the lamb, but at the same time states that they are called the same thing as each other and their Creator.

“The Lamb” by William Blake can be seen as an unabashed celebration of God's creation and all it contains. The little lamb of the poem's title is, for obvious reasons, not in a position to appreciate the wonders of this creation or the manifest beauties of the natural world in which he has the good fortune to live.

Fortunately, however, the speaker is on hand to tell the mute beast of the many wonderful things that the Almighty has done for him. Most importantly, He's given the lamb life and the food necessary to sustain it. He's also clothed the animal in fleece, the “Softest clothing wooly bright.” Not only that, but he's given the lamb a “tender voice” that makes “all the vales rejoice.” All in all, the lamb has much to be grateful for.

Although the poem is addressed to a lamb, it could easily be addressed to humanity as a whole, the vast majority of whose members remain blissfully unaware of, or indifferent to, the joys of God's creation. In that sense, most of us are “lambs” in that we need to be reminded of all that God has done for us and how fortunate we are to inhabit the world that he has created for us.

The main theme of the poem "The Lamb" by William Blake is praise for specific qualities of Jesus Christ and His gifts to humanity. In the first stanza, Blake asks the lamb if it knows who gave it life, soft wool, and a tender voice. In the second stanza, Blake reveals that Jesus Christ created the lamb with all of its positive qualities. Christ also referred to Himself as a lamb throughout the scriptures and became a "little child" when He came to earth to minister. William Blake then proceeds to praise Jesus's qualities by commenting on His meek and mild personality. Jesus is portrayed as a giving, loving, peaceful deity throughout the poem and Blake focuses on Christ's innocent attributes. Overall, Blake's poem praises the gifts from God and reveals his benevolence and tender qualities. 

In my opinion, the theme of this poem is Christian theology.  The poem is meant to convey to the listener some of the major ideas of the Christian faith.

The poem starts by asking the lamb if it knows who made it and who cares for it and gives it the things that it needs to live.  This is what is going on in the first stanza.

In the second stanza, the speaker tells the lamb (probably a symbol for a child) a bit about Jesus.  The speaker talks about how Jesus became human for the sake of all people.