What is the summary of William Blake's "Introduction" to Songs of Experience?

Since William Blake's Songs of Experience are best understood when compared to the corresponding Songs of Innocence, one should first read Introduction to Songs of Innocence before attempting a cogent summary of Introduction to Songs of Experience. The poem that introduces the first set of poems describes a fictional meeting with a child who requests the poet to first sing and then record in words his "songs of happy chear [sic]." Thus Blake introduces the first set of verses by describing them as "happy songs every child may joy to hear."
As he introduces the second set of poems, however, the poet isn't as clear about their purpose. Instead of responding to the request of a child, the poet gives a command to the reader to hear what the poet has to say. Instead of a cheerful piper, the writer of this section's poems presents himself as one who transcends time and was present at the Fall of man in the Garden of Eden. Just as the Creator called the "lapsed Soul" into renewed light, so the poet's songs in this section will offer a call to salvation. Thus the poet pleads, "O Earth O Earth return!" and "turn away no more."
However, it's important to understand that Blake isn't referring to the Fall as a fall from innocence into sin but from imagination and insight to the use of physical sight alone. To restore mankind to its ideal state, it will be necessary to look at the world not just through the optimistic simplicity of childhood, but through the mature understanding of one who can recognize both evil and good in the world. In "The Marriage of Heaven and Hell," Blake writes, "Without Contraries is no progression. Attraction and Repulsion, Reason and Energy, Love and Hate, are necessary to Human existence." Blake wants his Songs of Experience to provide the "Contraries" to the Songs of Innocence that will help readers progress toward a more imaginative and creative state of existence—the renewal and awakening the Introduction speaks of.
https://www.poetryfoundation.org/poems/43666/introduction-to-the-songs-of-experience

William Blake's "Introduction to Songs of Experience" exists as a "sister" poem to his "Introduction to Songs of Innocence." When looking at the poems together, one should notice the maturity to later "Experience" poems possesses over the earlier "Innocence" poems.
The poem in question calls for the reader to pay attention to him immediately: "Hear the voice of the Bard!" This attention is demanded based upon the "fact" the Bard knows of the past, present, and future. The poet goes on to declare that the reader should listen because the Bard has "walk'd among the ancient trees" and "heard the Holy Word" (meaning God has spoken to him while in the Garden of Eden).
In the second stanza, the speaker calls out for the reader, who has "fallen" from the light of God to give up the materialism of the earthly world ("the starry pole"). The third stanza calls out to the Earth, assumedly in the same position as man ("it," nature personified, has also turned from God).
The final stanza addresses both man and nature, both who the Bard calls out to "turn away no more." Yet, the speaker understands that both will still turn from God because both are too "slumberous" (weary, stubborn, or tainted) to do so.
In the end, the poem speaks to both man's and the earth's turning from God. The poem acts as a warning against turning from God, although the poet (in his mature experience) realizes that some, if not most, will be unable to do so.