For every nation We have made Mansak which they must follow; so let them not dispute with you on the matter, but invite them to your Lord. Verily, you indeed are on the straight guidance (67)And if they argue with you, say: "Allah knows best of what you do. (68)"Allah will judge between you on the Day of Resurrection about that wherein you used to differ. (69)

Allah tells us that He has made Mansak for every nation. Ibn Jarir said, "This means that there are Mansak for every Prophet's nation." He said, "The origin of the word Mansik in Arabic means the place to which a person returns repeatedly, for good or evil purposes. So the Manasik (rites) of Hajj are so called because the people return to them and adhere to them." If the phrase "For every nation We have ordained religious ceremonies" means that every Prophet's nation has its religious ceremonies as ordained by Allah, then the phrase "So let them (the pagans) not dispute with you on the matter" refers to the idolators. If the phrase "For every nation We have ordained religious ceremonies" means that it is the matter of Qadar (divine decree), as in the Ayah,

وَلِكُلٍّ وِجْهَةٌ هُوَ مُوَلِّيهَا

(For each nation there is a direction to which they face)[2:148] Allah says here:

هُمْ نَاسِكُوهُ

(which they Nasikuh) meaning, which they must act upon. The pronoun here refers back to those who have these religious ceremonies and ways, i.e., they do this by the will and decree of Allah, so do not let their dispute with you over that divert you from following the truth. Allah says:

وَادْعُ إِلَىٰ رَبِّكَ ۖ إِنَّكَ لَعَلَىٰ هُدًى مُّسْتَقِيمٍ

(but invite them to your Lord. Verily, you indeed are on the straight guidance.) i.e., a clear and straight path which will lead you to the desired end. This is like the Ayah:

وَلَا يَصُدُّنَّكَ عَنْ آيَاتِ اللَّهِ بَعْدَ إِذْ أُنزِلَتْ إِلَيْكَ ۖ وَادْعُ إِلَىٰ رَبِّكَ

(And let them not turn you away from the Ayat of Allah after they have been sent down to you: and invite to your Lord)[28:87]

وَإِن جَادَلُوكَ فَقُلِ اللَّهُ أَعْلَمُ بِمَا تَعْمَلُونَ

(And if they argue with you, say: "Allah knows best what you do.") This is like the Ayah:

وَإِن كَذَّبُوكَ فَقُل لِّي عَمَلِي وَلَكُمْ عَمَلُكُمْ ۖ أَنتُم بَرِيئُونَ مِمَّا أَعْمَلُ وَأَنَا بَرِيءٌ مِّمَّا تَعْمَلُونَ

(And if they belie you, say: "For me are my deeds and for you are your deeds! You are innocent of what I do, and I am innocent of what you do!")(10:41)

اللَّهُ أَعْلَمُ بِمَا تَعْمَلُونَ

(Allah knows best of what you do.) This is a stern warning and definite threat, as in the Ayah:

هُوَ أَعْلَمُ بِمَا تُفِيضُونَ فِيهِ ۖ كَفَىٰ بِهِ شَهِيدًا بَيْنِي وَبَيْنَكُمْ

(He knows best of what you say among yourselves concerning it! Sufficient is He as a witness between me and you!)(46:8). Allah says here:

اللَّهُ يَحْكُمُ بَيْنَكُمْ يَوْمَ الْقِيَامَةِ فِيمَا كُنتُمْ فِيهِ تَخْتَلِفُونَ

(Allah will judge between you on the Day of Resurrection about that wherein you used to differ.) This is like the Ayah:

فَلِذَٰلِكَ فَادْعُ ۖ وَاسْتَقِمْ كَمَا أُمِرْتَ ۖ وَلَا تَتَّبِعْ أَهْوَاءَهُمْ ۖ وَقُلْ آمَنتُ بِمَا أَنزَلَ اللَّهُ مِن كِتَابٍ

(So unto this then invite, and stand firm as you are commanded, and follow not their desires but say: "I believe in whatsoever Allah has sent down of the Book.")[42:15]