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.
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.
My suggestion is a bit different from the above post.
Though, on the surface level, lamb seems to stand just for innocence, but if analyzed deeply, the lamb in Blake's poem is a personal symbol which signifies God himself. The lamb indicates the softer aspects of God, for instance mercy, pity, forgiveness and so on. As the tiger of Blake's "The Tyger" from the experience poems, signifies the harsher and harder attributes of God, like wrath, rage, audaciousness etc., similarly "The Lamb" symbolizes some specific qualities of God himself.
This innocence of the lamb (the kindness of God) is a model for humankind. According to Blake, in order to bring peace and bliss, this innocence is very important. In fact, all his innocence and experience poems, in pairs, are foils for one another; and together, they make a sense.