Short answer
It depends on what is copied. Wearing another religion's symbols or religious dress, like a cross, a monk's habit or a festival costume worn to celebrate, is haram. Clothes that everyone now wears, such as jeans, suits and t-shirts, are allowed in the Hanafi view, as long as they meet the rules of awrah, gold, silk and modesty.
Tashabbuh (deliberate imitation) of non-Muslims in what is specific to their religion is haram, and done out of admiration for that religion it endangers faith. Imitating dress that is specific to them as a people is makruh. Clothing that has become common to all people is not imitation.
The four schools agree on this.
Explanation
The Prophet ﷺ said that whoever imitates a people is one of them. Ibn Abidin explains that this is not about every shared thing: Muslims and others eat, build and dress in many of the same ways. The blame falls on imitating what is blameworthy, or what is done in order to look like them in their distinct identity. The four schools agree on this principle.
Three levels.
- Religious symbols and dress: a cross, a priest's or monk's clothes, a sacred thread, or a costume worn to take part in another faith's festival. This is haram.
- Dress that marks a group: clothing that identifies a particular non-Muslim community, or a fashion known as the badge of a sinful scene. This is disliked and better avoided.
- Common clothing: shirts, trousers, jeans, suits, hoodies and trainers that people of every faith wear. This is allowed.
The necktie. Some earlier Deobandi scholars disliked it as a Western Christian garment. Mufti Taqi Usmani and others regard it as permissible today, since it no longer marks any religion, though they prefer that Muslims keep their own dress where they can.
Your own dress. Islam does not require Arab or South Asian clothes, but it values a visible Muslim identity. Whatever you wear, keep the rules in the dress code for men. For crosses on sports kits, see football shirts with crosses.
The evidence
Quran, Surah Al-Jaathiya 45:18
ثُمَّ جَعَلْنَٰكَ عَلَىٰ شَرِيعَةٍۢ مِّنَ ٱلْأَمْرِ فَٱتَّبِعْهَا وَلَا تَتَّبِعْ أَهْوَآءَ ٱلَّذِينَ لَا يَعْلَمُونَ ﴿١٨﴾
Then We put you, [O Muhammad], on an ordained way concerning the matter [of religion]; so follow it and do not follow the inclinations of those who do not know.
Muslims follow their own revealed way.
Sunan Abi Dawud 4031
Graded hasan sahih... من تشبه بقوم فهو منهم ...
"Whoever imitates a people is one of them."
The basis of the ruling on imitation.
Sahih Muslim 2077
Sahih... إن هذه من ثياب الكفار فلا تلبسها ...
"These are the clothes of the disbelievers, so do not wear them."
Dress that marked non-Muslims was forbidden to a Companion.
Where the ruling is recorded
- al-Durr al-Mukhtar with Radd al-Muhtar, Kitab al-Salah, chapter on what invalidates the prayer and what is disliked in it. Ibn Abidin: imitation is disliked only in what is blameworthy or intended as imitation.
- Takmila Fath al-Mulhim (Mufti Taqi Usmani), Kitab al-Libas wa al-Zina. The levels of imitation and the ruling on common clothing.
- Fatawa Mahmudiyya, Chapter on clothing and adornment. On wearing Western dress and the necktie.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.