“on your belly you will go.” The serpent in the Genesis record is not a literal snake, it is the figure hypocatastasis for the Devil [see commentary on Gen. 3:1, and also Appendix 14: “Names of the Devil,” under “serpent”]. In spite of that, however, God uses idioms that apply to a snake when talking to the Devil. Genesis 3:14 is a good example, because the idiom “to crawl on your belly” is an idiom for oppression and humiliation, and even today that kind of idiom is used (Ps. 44:25).
“you will eat dust.” This idiom refers to being defeated; “utter defeat.” We can see it being used in Psalm 72:9. We still use the same idiom for being defeated today, although usually in the context of being a loser in some kind of race or contest. The Devil, that “ancient serpent,” will always be defeated. He wins minor skirmishes, but he has been defeated over and over for thousands of years now, and eventually will be totally defeated and destroyed.