Deen means how you describe your Lord. If someone asks you, "Who is your Lord?" what would you say? When Pharaoh asked Prophet Ibrahim (A.S.), "Who is your Lord?" Prophet Ibrahim (A.S.) replied, "My Lord is the One who created the heavens and the earth, who gives us life and causes us to die, and who will bring us back to life after death."
This is the system of Ibrahim (A.S.), the system of Prophet Muhammad (S.A.W.), and it is also our system regarding our God. This is our argument. If any polytheist asks us, we should say that our Lord is the One who created the heavens, the earth, and everything in between.
As for the polytheists who worship idols, ask them about their idols — "Give us a certificate that this is truly your Lord." Do they have any certificate proving that their idol created the heavens and the earth? But we don’t need to provide any certificate. We simply say, "Our Lord is the One who created everything." That is why Allah told the Prophet (S.A.W.) that this is your system — the same as Ibrahim’s — and this is also my Deen: that I worship the Lord who created everything, and He is mighty, dominant, and wise.
This is our Deen.