Typically, our Christian missionary friends have been spinning this as to mean Allah was a pagan god as the pagan Arabs knew of the word “Allah” before the coming of the Prophet and Islam.
This simply highlights the Christian missionary’s/critic’s ignorance as the word Allah was initially used in a monotheistic sense by Ishmael (son of Abraham) and those of his followers. However, with time, these Arabs later slipped into paganism which ultimately led to the Arabs of Mecca leaving the pure belief of Allah being the ONLY God by making Allah a high god of their pantheon of gods.
The Prophet Muhammad (p) brought the Arabs back onto the worship of Allah alone with the introduction of Islam – the same beliefs Ishmael (p) introduced to Arabia.
A brief history of Meccan-Arab religious practice
We know that the word “Allah” was in use before the time of Prophet Muhammed (pbuh). If we read Ar-Raheeq ul-Makhtum we realise that the early Arabs did believe in Allah as the Only God. This is dated all the way back to the time of Prophet Ishmael (p) who resided in Makkah (Mecca) and learned Arabic as well as settling there . He preached the message of pure monotheism; “Most of the Arabs had complied with the call of Ishmael and professed the religion of his father Abraham. They worshipped Allah, professed His Oneness and followed His religion...” .
This shows that Allah was known as the Only God, just like the Muslims believe Him to be. Indeed Abraham and Ishmael (pbut) are considered to be Muslims, i.e. those who had submitted to the Will of the Only God, Allah.
How paganism was introduced after Ishmael and Abraham (pbut)
The issue of paganism came into the equation as the Arabs forgot this pure monotheism which was taught by Ishmael and his followers. The idolatry was originated from the actions of a man named Amr bin Luhai, he was known as a devoted and righteous man, well respected by his peers. However, after a trip away from Mecca he saw idol-worship in Syria.
Upon his return to the Meccans he introduced idol worship to the Meccans by bringing an idol named Hubal back from Syria and this resulted in the spread of a great many idols across Mecca.
They now (wrongly) believed Allah to be the high god of many gods
Despite the Meccan pagans’ acceptance of idols they still proclaimed belief in Allah in the sense that they saw Allah as the High God but used the idols as ‘lesser deities’ whom they believed “could intercede before Allah for the fulfilment of their wishes” .
Quite simply they had a pantheon of ‘gods’ but believed that Allah was the High God of their pantheon. 
The theory of Allah being considered as a High God is backed by Ar-Raheequl-Maktoum, Karen Armstrong  and W. M. Watt .
Effectively over the years they changed their belief in Allah, from the belief that Allah was the Only God (the Abrahamic teachings) to the belief that Allah was the High God of their many deities (pagan/polytheistic teachings).
There were 360 different idols, belonging to the pagans of Mecca, around the Ka’bah when Prophet Muhammed took charge of Mecca. These idols were subsequently broken, removed and burned under the authority of Prophet Muhammed .
Prophet Muhammad’s father - Abdullah
Another source that attests to the fact that the pre-Islamic Arabs used the name Allah and held a ‘belief’ in Him is the genealogy of Prophet Muhammed, his father’s name was actually Abdullah (meaning servant of Allah) . Interestingly enough, some of these pagan Arabs believed that Allah was the same God that the Jews and Christians worshipped 
Prophet Muhammad reintroduces Abrahamic monotheism
In 610 CE, the Prophet Muhammad began to call the pagan Arabs back to the pure monotheism which was taught by Abraham and Ishmael centuries earlier. By 632 CE the Meccan Arabs had given up their idols and returned back to pure monotheism – believing in Allah as the only god.
1. Ar-Raheequl-Makhtum by Safi-ur-Rahman Al-Mubarakpuri, Darussalam, 2002 pg 26-28
2. Ibid pg 45
3. Ibid pg 45-46
4. Ibid 46
5. Ibid 63
6. Islam a Short History by Karen Armstrong, Phoenix Press, 2001, pg 3
7. What Is Islam by W.Montgomery Watt, Longman Group, Second Edition, 1979, pg 47
History of paganism in Arabia
Ancient Arab Pagan Claims
Allah is not a moon god
Learn about Islam: