Surah al-Kafirun (The Disbelievers ) 109 : 6

Explanatory Note

Finally, the whole argument is summed up in the last verse.  Meaning that you, unbelievers, and I, Muhammad, are very far apart, without any bridge to connect us. This is a complete distinction and a precise, intelligible demarcation.

Such an attitude was essential then in order to expose the fundamental discrepancies in the essence, source and concepts of the two beliefs, i.e. between monotheism and polytheism, faith and unbelief. Faith is the way of life which directs man and the whole world towards God alone and determines for him the source of his religion, laws, values, criteria, ethics and morality. That source is God. Thus life proceeds for the believer, devoid of any form of idolatry. Idolatry on the other hand is the opposite of faith. The two never meet.

Jahiliyyah and Islam are two totally different entities, separated by a wide gulf. The only way to bridge that gulf is for jahiliyyah to liquidate itself completely and substitute for all its laws, values, standards and concepts their Islamic counterparts. The first step that should be taken in this respect by the person calling on people to embrace Islam is to segregate himself from jahiliyyah. He must be separated to the extent that any agreement or intercourse between him and jahiliyyah is absolutely impossible unless and until the people of jahiliyyah embrace Islam completely: no intermingling, no half measures or conciliation is permissible, however clever jahiliyyah may be in usurping or reflecting the role of Islam. The chief characteristic of a person who calls on others to adopt Islam is the clarity of this fact within himself and his solemn conviction of being radically different from those who do not share his outlook. They have their own religion, and he has his. His task is to change their standing point so that they may follow his path without false pretence or compromise. Failing this, he must withdraw completely, detach himself from their life and openly declare to them.

Without this basic separation confusion, double-dealing, doubt and distortion will certainly persist. And let it be clear in our minds here that the movement advocating Islam can never be constructed on any ambiguous or feeble foundations, but has to be built upon firmness, explicitness, frankness and fortitude as embodied in God’s instruction to us to declare

لَكُمۡ  Lakum is early - it's not deenukum lakum. It implies exclusivity. So what's being said is your way of life, your religion is only for you. It's never ever going to be for me. If lakum was after deenukum it wouldn't be "only." And wa leeya deen - and my deen is only for me when it comes to you. In other words, there's never going to be any exchange. 

 دِينِ  The deen at the end is actually deenee. For rhetorical reasons, the final ya is removed. Just a kasra left now. This kind of omission is used for several purposes. One of them is for exclamation. The vowel is not extended when we want to be emphatic. We don't say rabbee when we make du'a we say rabbi. [Ref: Nouman Ali Khan, Dream Tafsir Notes]