Ultima explains to Antonio that goodness has an inherent strength that always allows it to triumph, even in the face of overwhelming evil like Tenorio's family's witchcraft. In this passage, Anaya uses hyperbole and personification to intensify the weight of Ultima’s lesson, as the curandera says:
The smallest bit of good can stand against all the powers of evil in the world and it will emerge triumphant.
Ultima’s spiritual tradition treats good and evil as though they were living beings, able to deliberately influence the lives of humans. Here, her personification of good and evil as forces that "stand" and "emerge" creates the mental image of an active struggle between these two beings. By transforming abstract moral concepts into a hero and a villain who can fight each other, Ultima is making the lesson feel accessible and impactful to the childish Antonio. By giving good and evil these human-like qualities, Ultima also strengthens the idea that Antonio has a part to play in this battle. In her version of events, Antonio can fight on the side of “good” to “stand against all the powers of evil.”
The hyperbole in describing "the smallest bit of good" as capable of defeating "all the powers of evil" shows that Ultima believes goodness has immense potential to change things. Her deliberate exaggeration is an attempt to make Antonio see the significance of even the smallest virtuous acts. She is teaching him that moral courage is always worthwhile, no matter how futile it may seem.