Allah tells us how the disbelievers mocked the Messenger when they saw him. This is like the Ayah,

﴿وَإِذَا رَآكَ الَّذِينَ كَفَرُواْ إِن يَتَّخِذُونَكَ إِلاَّ هُزُواً﴾

(And when the disbelievers see you, they take you not except for mockery) (21:36), which means that they tried to find faults and shortcomings in him. Here Allah says:

﴿وَإِذَا رَأَوْكَ إِن يَتَّخِذُونَكَ إِلاَّ هُزُواً أَهَـذَا الَّذِى بَعَثَ اللَّهُ رَسُولاً ﴾

(And when they see you, they treat you only in mockery (saying): "Is this the one whom Allah has sent as a Messenger'') i.e., they said this by way of belittling and trying to undermine him, so Allah put them in their place, and said:

﴿وَلَقَدِ اسْتُهْزِىءَ بِرُسُلٍ مِّن قَبْلِكَ﴾

(And indeed Messengers before you were mocked at) (6:10)

﴿إِن كَادَ لَيُضِلُّنَا عَنْ ءَالِهَتِنَا﴾

(He would have nearly misled us from our gods,) They meant: `he nearly turned us away from worshipping idols, and he would have done so, had we not been patient and persevered in our ways.' So Allah said, warning and threatening them:

﴿وَسَوْفَ يَعْلَمُونَ حِينَ يَرَوْنَ الْعَذَابَ﴾

(And they will know, when they see the torment...)