William Blake was a man who experienced visions throughout his entire life, and this began when he was very young. Apparently, his first vision was when he was four, when, according to one source, he "saw God" when God "put his head to the window." This is something that so shocked him that he broke into screaming, much to the alarm of his parents. Yet this was not the end of his visions, and when he was eight or ten, Blake thought he saw a vision of "a tree filled with angels, bright angelic wings bespangling every bough like stars." His parents were greatly disconcerted by these visions, and his father in particular wanted to thrash his son after the latter vision.
However, it is clear that these visions greatly impacted his poetry. Consider the following line from "The Chimney Sweeper" in Blake's Songs of Innocence:
And by came an angel, who had a bright key,
And he opened all the coffins, and set them all free...
Note the juxtaposition of the miraculous and the divine with the everyday as a chimney sweep has a dream that offers him spiritual consolation against the grim hardships of his day to day reality. It is clear that Blake saw God through his visions in a very vivid and real way that allowed him to develop his own religious beliefs and views of a God who was much more intimately connected with the world and what went on in it than the church of his time believed in.