The main idea of "Prelude" is that as we grow older we grow more sophisticated in our views about the world.
This poem is autobiographical, and was intended to be Wordsworth's views on life when he was younger and the changes in his views as he got older.
Whate'er its mission, the soft breeze can come
To none more grateful than to me; escaped
From the vast city, where I long had pined
A discontented sojourner: now free,
Free as a bird to settle where I will.
As a child, the speaker enjoys nature and the simple things in life. Before religion and education, there was nature. The young see joy in simple things. When we get older, we get more sophisticated. We begin to look beyond ourselves, and beyond the simple pleasures that characterize our youth.