Final answer:
The word 'diabolic' in Wheatley's poem is inferred to mean devil-like, suggesting an evil quality. Wheatley uses this term to challenge the prejudiced view that associates blackness with moral inferiority and to assert racial equality.
Step-by-step explanation:
The word diabolic from its use in Phillis Wheatley's poem can be inferred to mean devil-like or characteristic of the devil. The term suggests an evil or malicious quality, and within the context of the poem, it conveys the racially charged perception that the skin color of African people was a sign of moral inferiority.
Wheatley challenges this notion by stating that regardless of skin color, all people, including those of her sable race, have the potential to be "refin'd, and join th' angelic train", thus asserting the equality of Black individuals and White Christians. Her use of the word diabolic in consequence serves to highlight the false and prejudiced attitudes held by some people of the time who associated blackness with sin or evil, attitudes which Wheatly aims to subvert and disprove.