My Teacher, the Messenger of Allah (saw) and his Students (Sahabah) already told us what it means!
azeezam adama sb, when and where did you see the final messenger of God or his companions yourself?
All information we have today is through other people and they told us whatever we have.
It is now our duty to find out what they told us is true or false, right or wrong. if we will not do that then we will remain confused forever. Just saying I believe this or that is neither here nor there. Whatever we claim to believe or practice must be based upon clear evidence and we should be able to prove it, not for ourselves alone but for others as well. Why? Because it is our duty to try and convince each other so that all human being could become one people under rule of law of God.
Now you don't need me to prove these points to you from the quran if you have read and understood the quran, do you? This is why we discuss things with each other and this is why God sent books and his messengers. Since there are no more books or messengers to come from God it is our duty to get involved in this missionary work.
In my view the quran is a book of deen not mazhab. This is why mazhab based beliefs and practices have nothing at all to do with deen of islam. Mazhab wastes peoples' time and energy and has no solutions for problems of humanity.
You can pray day and night all you like but it is not going to put food on your table. You can fast all your life but it is not going to put clothes on your back. You can recite first kalima day and night, it is not going to cure your mental and physical problems. You can visit kaaba day and night, it is not going to address any of your worldly needs. Even charity is just tokenism and cannot address human needs as it ought to.
How can such things be pillars of deen of islam? Pillars of islam are toheed and salaah ie unity of humanity under rule of law of one God as told by all his messengers to their people.
Pillar of deen of islam is SOWM ie holding back from thinking and doing anything which breaks the unity of humanity under rule of law of God.
Pillar of deen of islam is ZAKAAH. Thinking and doing everything one can that strengthens ummah or humanity under rule of law of God.
Pillar of deen of islam is HAJJ. Thinking and doing all one can to help establish kingdom of God in his kingdom ie undertaking the sacred journey towards the God set goals.
Tell me which of these does not fully address problems of humanity? This is why islam is deen not mazhab.
Think about it, why do we need a proper way of life in the first place? It is because we do not know how to live in this world properly. What is human life and living about? It is all about living and livelihood otherwise how can we live without food and drink and all the rest that goes with that? We are all fighting with each other all the time because we are trying to grab things of needs of each other. Why do we need things?
What is going to stop us from living like that? A proper constitution so that it could help us organise properly as a people. We need proper laws so that we could then regulate ourselves properly as a proper human community. If we have a constitution and its related laws then we need a kingdom wherein we could live by them or implement them.
Can you now see why islam is not a mazhab and it is addeen?
It was and it is stupidity of mullaans and their supporters who have been misleading and confusing this ummah for centuries. All because people themselves did not and do not bother to learn sense for making proper sense of things.
I hope I have made things clear enough for people of knowledge to understand why islam is addeen and not a mazhab.
Regards and all the best.