I have been discussing about some set of people about religion Islam they denied the basics by saying that it's not an religion but an Ideology.
Islam is a 'deen' i.e. a system with principles and laws that people follow. Allah says:
إِنَّ الدّينَ عِندَ اللَّهِ الإِسلامُ....
Indeed, the Deen with Allah is Islam... (Quran 3:19)
'Deen' is also generally used for any system, irrespective of being religious or not.
Allah uses it to describe the kings law that Prophet Yousef ruled under:
فَبَدَأَ بِأَوعِيَتِهِم قَبلَ وِعاءِ أَخيهِ ثُمَّ استَخرَجَها مِن وِعاءِ أَخيهِ ۚ كَذٰلِكَ كِدنا لِيوسُفَ ۖ ما كانَ لِيَأخُذَ أَخاهُ في دينِ المَلِكِ إِلّا أَن يَشاءَ اللَّهُ ۚ نَرفَعُ دَرَجاتٍ مَن نَشاءُ ۗ وَفَوقَ كُلِّ ذي عِلمٍ عَليمٌ
"Then he began the search with their bags before his brother's bag, then he produced it from his brother's bag. Thus did We contrive for Joseph. He could not have taken his brother according to the king's law/ʾakhaahu fee deeni l-maliki unless God willed. We raise in rank whomever We please, and above every man of knowledge is One who knows best" (Quran 12:76)