@lizfeehily,
William Blake wrote a poem entitled "The Lamb," among a series of poems describing god's creation. I got a copy of it for you:
Little Lamb who made thee
Dost thou know who made thee
Gave thee life & bid thee feed.
By the stream & o’er the mead;
Gave thee clothing of delight,
Softest clothing wooly bright;
Gave thee such a tender voice,
Making all the vales rejoice!
Little Lamb who made thee
Dost thou know who made thee
Little Lamb I’ll tell thee,
Little Lamb I’ll tell thee!
He is called by thy name,
For he calls himself a Lamb:
He is meek & he is mild,
He became a little child:
I a child & thou a lamb,
We are called by his name.
Little Lamb God bless thee.
Little Lamb God bless thee.
The reference is to Jesus, who, in christian iconography, is often portrayed symbolically as a lamb, and often called the Lamb of God.
Homely can mean either unattractive, or it can mean cozy, comfortable, like one's own home. Munro means it in the second sense here. The sentiment referred to is that type of christianity which reflects the meek, mild, loving and simple version of Jesus, a gentle redeemer. This is the "wholesome gentle-hearted creed" referred to earlier. Munro is linking the image of the Lamb of God to this wholesome gentle-hearted creed, to create an idea of the beautiful homely Christ child sentiment. It is christianity as a creed of love and acceptance.