There is no onomatopoeia (strictly) in this excerpt. Onomatopoeia in the most restrictive sense is a word which resembles or evokes a sound in a directly sonic way. For example, if the sound of a car moving past quickly has a certain windy sound, people might find the word “whoosh” sonically approximates that sound. The word “crack” refers to a sound phenomenon semantically. It means a certain sound in the voice, rather than sounding like that sound.
The most relevant poetical device to characterise the sentence is probably metaphor. An intuitive way of thinking about metaphor is anything that is not literally true. The character says they will not crack the voice of God’s law. But God’s law is a law, and a law doesn’t have a voice. What can the speaker mean by cracking the voice of a law? They mean they will not make the law appear less magnificent and authoritative than it is, in the way that a person speaking nervously makes a message seem less dominant.
You can also use this strong utterance as an opportunity to consider the psychology of the character and the themes of the work.