Tafsir of Al-Anbiya' 21:34

"And We did not grant to any human before you immortality..."

They used to estimate that he [the Prophet] would die, and they would gloat over his death. So, Allah the Exalted negated this gloating from him.

"So if you die, would they be the immortals?"

This means: Allah has decreed that no human shall be immortal in this world. Neither you nor they; all are subject to death. If this is the case, then if you die, will they remain? The meaning is captured in the words of the poet:

Say to those who gloat over our misfortune: Wake up! The gloaters shall surely encounter what we have encountered.

"And We test you with evil and good as a trial..."

Meaning: We test you with calamities that require patience, and with blessings that require gratitude.

"...and to Us you will be returned."

We will recompense you according to the patience or gratitude found in you.

He called this a "trial" (ibtila')—even though He is All-Knowing of what the workers will do before they even exist—because it is in the form of a test.

"And [as] a trial..." (fitnatan)

This is an infinitive (masdar) that emphasizes the meaning of "We test you" (nablukum), though it is not derived from the same root.

"And when those who disbelieved see you, they take you not except in mockery: 'Is this the one who mentions your gods?' And they are, at the mention of the Most Merciful, disbelievers."