Surah 24. An-Nur, Ayah 63

I.e., his summons to God's message in general, spoken of in verses {46-54} above, as well as to a particular course of communal action, referred to in verse {62}. Alternatively, "the Apostle's summons" may, in this context, be synonymous with the Qur'an itself.

 There is also a different interpretation: “Do not call the Messenger ˹by his name˺ as you call one another.” In other words, Do not say, ‘O Muḥammad.’ Rather, say, ‘O Prophet.’

 Some hypocrites used to sneak out while the Prophet (ﷺ) was talking.

Three significations are possible. One is that adopted in the Translation, which agrees with the view of most Commentators. Another would be: 'Do not think that the prayer of the Prophet of Allah is like your ordinary requests to another: the Prophet's prayer will be about serious matters and will be accepted by Allah'. A third interpretation would be: 'Do not address the Prophet familiarly as you would address one another: use proper terms of respect for him.'

The "trial" is understood to be some misfortune in this life, and the "grievous Penalty" to be the punishment in the Hereafter.