الدين (read: ad-din) usually translated as religion. But Arabic words always have various meaning, and الدين can be translated as debt. It is called a debt because we cannot pay for it however we try to do many good deeds and serving Allah as well. Everything that have been created by Allah for human are free to enjoyed, could you imagine that if Allah asking for payment for every breath that we take, every step that we made, every beat that our heart done, and so on. How could we pay all those things.
While Allah only asking us to worship Him and join none with Him in worship. It can be treated as the way we pay our debt to Allah. He questioning the people who seek other than the religion of Allah, while to Him submitted all creatures in the heavens and the earth, willingly or unwillingly. And the religion of Allah is Islam, as what have been approved for us. For he who desires other than Islam as religion – never will it be accepted from him, and he, in the Hereafter, will be among the losers.
Even though we cannot pay any of our debt to Allah, we should still worshiping Him alone and doing good deeds as He commanded and avoid any evil deeds that He forbade. It is the only way to get His mercy, luckily for us if we not to go to the hell fire.