وَقَدْ كَفَرُوا بِهِ مِن قَبْلُ
...
Indeed they did disbelieve before,
means, how can they attain faith in the Hereafter when in this world they disbelieved in the truth and denied the Messengers?
...
وَيَقْذِفُونَ بِالْغَيْبِ مِن مَّكَانٍ بَعِيدٍ
and they (used to) conjecture about the Unseen from a far place.
Malik narrated from Zayd bin Aslam that he said:
وَيَقْذِفُونَ بِالْغَيْبِ
(and they (used to) conjecture about the Unseen),
means, "By guessing." as Allah says,
رَجْماً بِالْغَيْبِ
guessing at the Unseen. (18:22)
Sometimes they said he was a poet, sometimes they said he was a soothsayer, or a sorcerer, or a mad man, or other baseless comments. They denied the idea of resurrection and said:
إِن نَّظُنُّ إِلاَّ ظَنّاً وَمَا نَحْنُ بِمُسْتَيْقِنِينَ
we do not think it but as a conjecture, and we have no firm convincing belief. (45:32)
Qatadah and Mujahid said,
"Their belief that there would be no resurrection, no Paradise and no Hell was based on mere conjecture."