Evidence for the word "Allah" comes from the Christian roots.

There is ample evidence that Muhammad acquired the knowledge of Allah from Christian Bishop Waraqa ibn Nawfal of the Nestorian Church from Syria. Muhammad was trained by his wealthy wife Khadija who was a former employer of Muhammad. He learned much from Khadija for 9 years from 610 to 619AD.

The root of Is-lam starts from Christianity as Bishop Waraqa was a Christian. Moslem has hijacked the Hebrews Prophets and islamized them into Moslem. This is a crime against the Christian roots from the Hebrew Abraham to Yahushua aka Jesus the Son of Yahuweh Alaha.

Yahu'shua is equal in authority to Yahu'weh Allaha for they are One: "I and My Father are One." -- John 10:30 Nothing can stop Christians from the use of Allah which is originally belong to the Christians who called Elohim in Hebrew, Allaha in Aramaic, God in English and Allah in Arabic prior to AD33.

The Day of Pentecost was 50 days after the resurrection of Yahu'shua from His death i.e. the Passover Feast or 10 days after the ascension of Yahu'shua the Messiah as He remained around on earth for 40 days after His resurrection.

Acts 2:5 And there were dwelling at Jerusalem Jews, devout men, out of every nation under heaven.
Acts 2:6 Now when this was noised abroad, the multitude came together, and were confounded, because that every man heard them speak in his own language.
Acts 2:7 And they were all amazed and marvelled, saying one to another, Behold, are not all these which speak Galilaeans?
Acts 2:8 And how hear we every man in our own tongue, wherein we were born?
Acts 2:9 Parthians, and Medes, and Elamites, and the dwellers in Mesopotamia, and in Judaea, and Cappadocia, in Pontus, and Asia,
Acts 2:10 Phrygia, and Pamphylia, in Egypt, and in the parts of Libya about Cyrene, and strangers of Rome, Jews and proselytes,
Acts 2:11 Cretes and Arabians, we do hear them speak in our tongues the wonderful works of Elohim.

The Arabians are the Arabs from Arabia and they have used "Allah" in Arabic or Elohim in Hebrew for "God" around AD33.

Now Moslem has turned their back against the Christians just as Satan works behind them. Moslem please study your historical roots of Is-lam and produce 4 eye-witnesses to prove that Muhammad never learn anything about Allah from Bishop Waraqa if you are truthful. Waraqa knew Muhammad since he was a young boy. Khadija was a cousin of Waraqa.

So unless Moslem can prove that Khadija did not bring Muhammad to consult and learn from Bishop Waraqa about Allah and Gabriel by 4 eye-witnesses vide Quran Sura 24:13, there is nothing to refrain the Christians from the use of Allah but have to proclaim that Allaha come from prior learning from Christian Bishop Waraqa instead.