Is there anything wrong with visiting churches for sightseeing?
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Tawhīd (The Oneness of Allah) is the founding brick of Islam. Tawhīd is the soul of one’s Islam. It is the root to the tree of Imān.
Tawhīd demands one to acknowledge with certainty that the only entity worth of worship is One Allah. Everything besides Allah Ta’ālā is not deserving of worship or reverence. Tawhīd unshackles one from the slavery and servitude to all others.
A Muslim is one who is buzzing and overflowing with the love of Allah Ta’ālā. His pledge and loyalty to Allah Ta’ālā radiates from all his actions. A True Muslim is one who is disturbed by watching other than One Allah worshipped.
Allah constantly showers us with His favours and blessings. We enjoy the countless blessings of Allah; He clothes us, He satiates our hunger, He quenches our thirst, He fulfils our needs and protects us. How can we ever have any affiliation to something which opposes Allah?
A True Muslim is sympathetic and aspires guidance for all those who have fallen prey to false idols and deities. After all, we have been sent to free man from the servitude of the creation of Allah, and introduce them to the freedom of servitude to One Allah.
Structures and buildings associated to teaching, preaching and worshipping other than Allah Ta’ālā are hotbeds of Shaytān. They are not worthy of adoration or reverence. Shaytān is idolised and worshipped in such places. Tawhīd is martyred. Thus, the Jurists have mentioned it is severely disliked for a Muslim to visit and enter the edifices and structures of other faiths.
And Allah Knows Best
Mufti Faraz al-Mahmudi
 وَفِي التَّتَارْخَانِيَّة يُكْرَهُ لِلْمُسْلِمِ الدُّخُولُ فِي الْبِيعَةِ وَالْكَنِيسَةِ، وَإِنَّمَا يُكْرَهُ مِنْ حَيْثُ إنَّهُ مَجْمَعُ الشَّيَاطِينِ لَا مِنْ حَيْثُ إنَّهُ لَيْسَ لَهُ حَقُّ الدُّخُولِ اهـ قَالَ فِي الْبَحْرِ: وَالظَّاهِرُ أَنَّهَا تَحْرِيمِيَّةٌ؛ لِأَنَّهَا الْمُرَادَةُ عِنْدَ إطْلَاقِهِمْ، (رد المحتار ج 1 ص 380 أيج أيم سعيد)