Every Nation has its Religious Ceremonies
Allah tells;
لِكُلِّ أُمَّةٍ جَعَلْنَا مَنسَكًا هُمْ نَاسِكُوهُ فَلَ يُنَازِعُنَّكَ فِي الاَْمْرِ
For every nation We have made Mansak which they must follow; so let them not dispute with you on the matter,
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 إِلَى رَبِّكَ إِنَّكَ لَعَلَى 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 ءَايَـتِ اللَّهِ بَعْدَ إِذْ أُنزِلَتْ إِلَيْكَ وَادْعُ إِلَى 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