A figure of speech is a type of language that is not literal. In this case, the poem takes on many meanings. It is common to use figurative language like metaphors in a sonnet. This beautiful sonnet uses metaphors, personification, and allusion.
The strongest metaphor is that God is a person. God is personified, seen as stooping and having an “awful brain.” There is also an allusion to Sisyphus, a figure from Greek mythology. Humans are compared to Sisyphus, and their life is compared to “struggle up a never-ending stair.”
Cullen uses metaphor when describing how he tries to comprehend the word of God.
Inscrutable His ways are, and immune
To catechism by a mind too strewn
With petty cares to slightly understand
The speaker describes his mind as a place and ideas as things that can literally be strewn around.
Another metaphor is in the last line.
Yet do I marvel at this curious thing:
To make a poet black, and bid him sing!
In this case, writing a poem is compared to singing. The poem and the song are both an expression of beauty.