Tafsir of Az-Zukhruf 43:65

Then the factions—the groups that formed parties—differed from among them—that is, from among those to whom he was sent and whom he addressed, namely the Jews and the Christians, who are the people of his call, peace be upon him. It is also said that what is meant are the Christians, who are the people of his response, peace be upon him, and they differed into sects: Melkites, Nestorians, and Jacobites.

"So woe to those who have wronged" from among the differing ones, and they are those who did not say: "He is the servant of Allah and His messenger."

"...from the punishment of a painful day."

This is the Day of Resurrection, and "painful" is an adjective for the punishment, or it is an adjective for the day, based on metaphorical attribution.