Other translatios for العروة الوثقي include:

firm handle
firm stead
firm tie
frim bond

Ref: Concise Islamic Dictionary, Hamdy Ebeid

Also

Firmest hand hold

Ref: A Dictionary of Islamic terms, Dr. Muhammad Ali Alkhuli

this is an almost literal translation. It might need some refining.

العروة is a loop shaped strap that a person holds on to. You hang on one when you stand in a bus, you pull upon one when you are mounting your camel. But more seriously, someone may throw one to you if you are drifting in turbulent waters, this one you hold on to until you are pulled to safety.

You can translate الدين as the religion, but the context is most likely Islamic text in which "the religion" refers to Islam.

Here is of one of several ayat in the Quran mentioning the term:

وَمَنْ يُسْلِمْ وَجْهَهُ إِلَى اللَّهِ وَهُوَ مُحْسِنٌ فَقَدِ اسْتَمْسَكَ بِالْعُرْوَةِ الْوُثْقَى وَإِلَى اللَّهِ عاقبة الأمور.

The translation from Yusuf Ali:

Whoever submits his whole self to Allah, and is a doer of good, has grasped indeed the most trustworthy hand-hold; and with Allah rests the End and Decision of (all) affairs

Other translations can be handhold (the hyphen is not necessary), grip, or handgrip.

Hope this helps.

Alaa Zeineldine