What Religion Existed Before Islam?
Question

Was there a religion called Islam before the Prophet (peace and blessings of Allah be upon him) came?
Praise be to Allah.General meaning of Islam
It should be noted that the word “Islam ” may refer to two things: a general meaning and a specific meaning. 
Islam in a general sense means submitting to Allah Alone, which implies worshipping Him Alone and obeying Him Alone. 
Everyone who believes in Allah Alone and worships Him according to His laws that have not been abrogated is a Muslim. 
Shaykh al-Islam said in al-Tadmuriyyah, p. 168-170: 
“This religion is the religion of Islam , besides which Allah will accept no other religion, either from the earlier or the later generations. All the Prophets followed Islam. Allah says of Nuh (interpretation of the meaning): 
“And recite to them the news of Nuh (Noah). When he said to his people: ‘O my people, if my stay (with you), and my reminding (you) of the Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.) of Allah is hard on you, then I put my trust in Allah. So devise your plot, you and your partners, and let not your plot be in doubt for you. Then pass your sentence on me and give me no respite.
72. ‘But if you turn away (from accepting my doctrine of Islamic Monotheism, i.e. to worship none but Allah), then no reward have I asked of you, my reward is only from Allah, and I have been commanded to be of the Muslims (i.e. those who submit to Allah’s Will).’” [Yunus 10:71-72]
And Allah says of Ibrahim (interpretation of the meaning): 
“And who turns away from the religion of Ibrahim (Abraham) (i.e. Islamic Monotheism ) except him who befools himself? Truly, We chose him in this world and verily, in the Hereafter he will be among the righteous. When his Lord said to him, ‘Submit (i.e. be a Muslim)!’ He said, ‘I have submitted myself (as a Muslim) to the Lord of the ‘Alamin (mankind, jinn and all that exists).’ And this (submission to Allah, Islam) was enjoined by Ibrahim (Abraham) upon his sons and by Ya’qub (Jacob) (saying), ‘O my sons! Allah has chosen for you the (true) religion, then die not except in the Faith of Islam (as Muslims — Islamic Monotheism).’” [al-Baqarah 2:130-132]
And He says of Musa (interpretation of the meaning): 
“And Musa (Moses) said: ‘O my people! If you have believed in Allah, then put your trust in Him if you are Muslims (those who submit to Allah’s Will).’” [Yunus 10:84]
And He says of the Messiah (interpretation of the meaning): 
“And when I (Allah) inspired Al-Hawariyyun (the disciples) [of ‘Isa (Jesus)] to believe in Me and My Messenger, they said: ‘We believe. And bear witness that we are Muslims.’” [al-Maidah 5:111]
And He says of the previous Prophets (interpretation of the meaning): 
“…by which the Prophets, who submitted themselves to Allah’s Will …” [al-Maidah 5:44]
And He tells us that Bilqis said (interpretation of the meaning): 
“My Lord! Verily, I have wronged myself, and I submit [in Islam, together with Sulayman (Solomon)] to Allah, the Lord of the ‘Alamin(mankind, jinn and all that exists).” [al-Naml 27:44]
This is the religion of Islam , besides which Allah does not accept any other. This means obeying Allah at any given time by doing what He commanded at that time. So initially the command was to face towards the Rock [in Jerusalem] in prayer, then the command came to face towards the Ka’bah. Both actions come under the heading of the religion of Islam, because religion is obedience and worship in both cases, but the actions varied in some regards which was the direction faced in prayer. Similarly the religion of all the Messengers is one and the same, but their laws varied.” 
With this general meaning of the word “Islam ” in mind, it is correct to say that there was a religion called Islam before the Prophet Muhammad (peace and blessings of Allah be upon him) was sent. 
All the Prophets who were sent were Muslims, and their followers who adhered to their true religion, before it was distorted or abrogated, were Muslims. 
What religion existed before Prophet Muhammad?
Before the Prophet (peace and blessings of Allah be upon him) came there were a few individuals who followed the true religion with which Allah had sent ‘Isa (peace be upon him). This is indicated by the report narrated by Muslim (2865) from ‘Iyad ibn Himar al-Mujashi’i, that the Messenger of Allah (peace and blessings of Allah be upon him) said one day in his khutbah: “Allah looked at the people of the earth and was angry with them all, Arabs and non-Arabs, except for the remainder of the People of the Book. And He said: ‘I have only sent you to test you and test others by you…’”  
Al-Nawawi (may Allah have mercy on him) said: 
Maqt (translated here as “angry”) means extreme anger. Looking refers to the time before the Messenger of Allah (peace and blessings of Allah be upon him) was sent. What is meant by the remainder of the People of the Book is those who continued to adhere to their true religion without altering it. End quote. 
Specific meaning of Islam
With regard to Islam in a specific sense, this is the religion with which Allah sent His Messenger Muhammad (peace and blessings of Allah be upon him). 
In this specific sense, it is not correct to say that there was a religion called Islam before the Prophet Muhammad (peace and blessings of Allah be upon him) was sent. 
Shaykh al-Islam said in al-Tadmuriyyah (p. 173-174): 
“People have disputed concerning those who came before them of the nations of Musa and Ibrahim – were they Muslims or not? 
This is a semantic dispute, because Islam in the specific sense with which Allah sent Muhammad (peace and blessings of Allah be upon him) and the law of the Quran is not followed by anyone except the ummah of Muhammad (peace and blessings of Allah be upon him). This is what is meant by Islam nowadays. But with regard to Islam in a general sense, referring to every law with which Allah sent one of His prophets, this includes the Islam of every nation that followed one of the prophets.” 
And Allah knows best.