"I convey unto you the Messages of my Lord and give sincere advice to you. And I know from Allah what you know not."
This is the attribute of a Messenger, that he conveys using plain, yet eloquent words, offers sincere advice and is knowledgeable about Allah; indeed, no other people can compete with the Prophets in this regard.
In his Sahih, Muslim recorded that the Messenger of Allah said to his Companions on the Day of Arafah, when their gathering was as large as it ever was,
أَيُّهَا النَّاسُ إِنَّكُمْ مَسْوُولُونَ عَنِّي فَمَا أَنْتُمْ قَايِلُونَ
O people! You will be asked about me, so what will you say?
They said, "We testify that you have conveyed and delivered (the Message) and offered sincere advice."
So he kept raising his finger to the sky and lowering it towards them, saying,
اللَّهُمَّ اشْهَدْ اللَّهُمَّ اشْهَد
O Allah! Bear witness, O Allah! Bear witness.