Allah swears by His creation, in which some of His signs can be seen in His creatures. These also indicate the perfection of His Names and Attributes. He then swears by the hidden things that they cannot see. This is an oath swearing that the Qur'an is His Speech, His inspiration and His revelation to His servant and Messenger, whom He chose to convey His Message, and the Messenger carried out this trust faithfully. So Allah says,
فَلاَ أُقْسِمُ بِمَا تُبْصِرُونَ - وَمَا لاَ تُبْصِرُونَ - إِنَّهُ لَقَوْلُ رَسُولٍ كَرِيمٍ
(So I swear by whatsoever you see, and by whatsoever you see not, that this is verily the word of an honored Messenger.) meaning, Muhammad ﷺ. Allah gave this description to him, a description which carries the meaning of conveying, because the duty of a messenger is to convey from the sender. Therefore, Allah gave this description to the angelic Messenger in Surat At-Takwir, where he said,
إِنَّهُ لَقَوْلُ رَسُولٍ كَرِيمٍ ذِى قُوَّةٍ عِندَ ذِى الْعَرْشِ مَكِينٍ مُّطَـعٍ ثَمَّ أَمِينٍ
(Verily, this is the Word of (this Qur'an brought by) a most honorable messenger. Owner of power (and high rank) with Allah, the Lord of the Throne. Obeyed and trustworthy.) (81:19-21) And here, it refers to Jibril. Then Allah says,
وَمَا صَـحِبُكُمْ بِمَجْنُونٍ
(and your companion is not a madman. ) (81:22) meaning, Muhammad ﷺ.
وَلَقَدْ رَءَاهُ بِالاٍّفُقِ الْمُبِينِ
(And indeed he saw him in the clear horizon.) (81:23) meaning, Muhammad ﷺ saw Jibril in his true form in which Allah created him.
وَمَا هُوَ عَلَى الْغَيْبِ بِضَنِينٍ
(And he is not Danin with the Unseen.) (81:24) meaning, suspicious.
وَمَا هُوَ بِقَوْلِ شَيْطَـنٍ رَّجِيمٍ
(And it (the Qur'an) is not he word of the outcast Shaytan.) (81:25) This is similar to what is being said here.
وَمَا هُوَ بِقَوْلِ شَاعِرٍ قَلِيلاً مَّا تُؤْمِنُونَ - وَلاَ بِقَوْلِ كَاهِنٍ قَلِيلاً مَّا تَذَكَّرُونَ
(It is not the word of a poet, little is that you believe! Nor is it the word of soothsayer, little is that you remember!) So in one instance Allah applies the term messenger to the angelic Messenger and in another instance He applies it to the human Messenger (Muhammad ﷺ). This is because both of them are conveying from Allah that which has been entrusted to them of Allah's revelation and Speech. Thus, Allah says,
تَنزِيلٌ مِّن رَّبِّ الْعَـلَمِينَ
(This is the revelation sent down from the Lord of all that exists.)