"Inna alDina 'inda Allahe Al-Islam" is the beginning part of the verse 3:19 of the Holy Quran. Abdullah Yusuf Ali translates the whole verse as follows:
003.019 The Religion before God is Islam: Nor did the People of the Book dissent therefrom except through envy of each other, after knowledge had come to them. But if any deny the Signs of God, God is swift in calling to account.
The Arabic word Deen in the above verse is a comprehensive term referring to a way of life including Faith, mode of worship, rituals and principles we follow in our individual life and the society. Islam is full obedience and submission to God and God alone.
The following verses explain the concept of the religion of the prophets:
002.127 And remember Abraham and Isma'il raised the foundations of the House (With this prayer): "Our Lord! Accept (this service) from us: For Thou art the All-Hearing, the All-knowing.
002.128 "Our Lord! make of us Muslims, bowing to Thy (Will), and of our progeny a people Muslim, bowing to Thy (will); and show us our place for the celebration of (due) rites; and turn unto us (in Mercy); for Thou art the Oft-Returning, Most Merciful.
002.129 "Our Lord! send amongst them an Apostle of their own, who shall rehearse Thy Signs to them and instruct them in scripture and wisdom, and sanctify them: For Thou art the Exalted in Might, the Wise."
002.130 And who turns away from the religion of Abraham but such as debase their souls with folly? Him We chose and rendered pure in this world: And he will be in the Hereafter in the ranks of the Righteous.
002.131 Behold! his Lord said to him: "Bow (thy will to Me):" He said: "I bow (my will) to the Lord and Cherisher of the Universe."
002.132 And this was the legacy that Abraham left to his sons, and so did Jacob; "Oh my sons! God hath chosen the Faith for you; then die not except in the Faith of Islam."
002.133 Were ye witnesses when death appeared before Jacob? Behold, he said to his sons: "What will ye worship after me?" They said: "We shall worship Thy God and the God of thy fathers, of Abraham, Isma'il and Isaac,- the one (True) God: To Him we bow (in Islam)."
002.134 That was a people that hath passed away. They shall reap the fruit of what they did, and ye of what ye do! Of their merits there is no question in your case!
002.135 They say: "Become Jews or Christians if ye would be guided (To salvation)." Say thou: "Nay! (I would rather) the Religion of Abraham the True, and he joined not gods with God."
002.136 Say ye: "We believe in God, and the revelation given to us, and to Abraham, Isma'il, Isaac, Jacob, and the Tribes, and that given to Moses and Jesus, and that given to (all) prophets from their Lord: We make no difference between one and another of them: And we bow to God (in Islam)."
002.137 So if they believe as ye believe, they are indeed on the right path; but if they turn back, it is they who are in schism; but God will suffice thee as against them, and He is the All-Hearing, the All-Knowing.