There is only one theological reality which has been the core of all the Prophets’ teachings from Adam until Prophet Muhammad; that there is no deity worthy of worship except God and that God, the Creator of all, has concerned Himself with His creation by sending them messengers commissioned by Him with His message.
This obliges all of humankind to follow the teachings of the Prophet(s) sent to them throughout the ages. God has stated “and We have sent no messenger before you, but We inspired him, (saying): there is no God save Me (God), so worship Me.” , and “and We never sent a messenger except with the language of his people, so that he might make (the message) clear for them.” , and “Serve God, you have no other God save Him.” Therefore it should be clear that total worship of God is the core of the Islamic teaching and likewise the core of all previous religious traditions.
Islam is also similar to previous religious traditions in that it has outlawed the same major sins such as adultery and homicide. Another area of similarity is in the general acts of worship such as prayer and alms giving. What has changed, though, is the manner in which these are to be preformed.
In this light Islam has complemented previous revelations and has completed the message of the Divine to all of humankind:
He has ordained for you that religion which He commended unto Noah, and that which We inspire in you (Muhammad), and that which We commended unto Abraham and Moses and Jesus, saying: establish the religion, and be not divided. Dreadful for the idolaters is that which you call them to. God chooses for Himself whom He will, and guides unto Himself he who turns (toward Him).” , and “and who forsakes the religion of Abraham save him who fools himself ? Verily We chose him in the world, and indeed in the Hereafter he is among the righteous.
When his Lord said unto him: Surrender! he said: I have surrendered to the Lord of the Worlds.The same did Abraham enjoin upon his sons, and also Jacob, (saying): O my sons! Indeed God has chosen for you the (true) religion; therefore die not save as men who have surrendered (unto Him).” , and “I am commanded to among those who submit to God.” , and “And Moses said: O my people! If ye have believed in God then put trust in Him, if ye have indeed surrendered (unto Him)!”
And God says in another verse And who would be averse to the religion of Abraham except one who makes a fool of himself. And We had chosen him in this world, and indeed he, in the Hereafter, will be among the righteous.When his Lord said to him, "Submit", he said "I have submitted [in Islam] to the Lord of the worlds."And Abraham instructed his sons [to do the same] and [so did] Jacob, [saying], "O my sons, indeed Allah has chosen for you this religion, so do not die except while you are Muslims." (2.130-132)
Also God says “…. and I have been commanded to be of the Muslims." Furthermore God quoted Belquis, the Queen of Sheba saying “She said, "My Lord, indeed I have wronged myself, and I submit with Solomon to Allah , Lord of the worlds."
From all the above mentioned verses God indicated that Abraham, Ishmael, Isaac, Jacob and the Israelites, Noah, Moses, Jesus, Solomon and their followers are all Muslims. This means that Islam was the only religion since the coming of Adam until the Prophet Muhammad.