Al-An'am: 34
{And messengers have already been denied...}
This is a consolation for the Messenger of Allah (ﷺ). It serves as evidence that His saying: {For indeed, they do not deny you} (Al-An'am: 33) is not a negation of their denial of him. Rather, it is like your saying to your servant: "They did not insult you, but they insulted me."
{...for what they were denied and harmed}
Meaning: because of their denial and the harm they suffered.
{...and there is no changer of the words of Allah}
Meaning: no changer of His promises, as in His saying: {And Our word has already preceded for Our servants, the messengers, that they would indeed be those given victory} (As-Saffat: 171).
{...and there has already come to you some of the information about the messengers}
Meaning: some of their news, their stories, and the hardships they endured in patiently bearing the polytheists.
{But if their aversion is difficult for you, then if you can seek a tunnel into the earth or a stairway into the sky to bring them a sign, [then do so]. But if Allah had willed, He would have gathered them upon guidance. So never be among the ignorant. Only those who hear will respond. But the dead - Allah will resurrect them; then to Him they will be returned.}