Surah 38. Sad, Ayah 8

Lit., "that they are in doubt of": i.e., it is not the personality of the Prophet that fills them with distrust, but, rather, the substance of the message proclaimed by him - and, in particular, his insistence on God's absolute oneness and uniqueness, which runs counter to their habits of thought and social traditions.

Sc., "on people who refuse to accept the truth".

 In other words, they do not question the honesty of the Prophet (ﷺ), but question the Quran itself. This verse is similar to 6:33.

They have no clear idea of how Allah's Message comes! It is not a worldly thing to be given to any one. It is a divine thing requiring spiritual preparation. If they close their eyes to it now, it will be brought home to them when they taste the consequences of their folly!

Here comes in envy. 'If a Message had to come, why should it come to him, the orphan son of 'Abdullah, and not to one of our own great men?'