Why then, in order to become a Muslim one must declare that Allah is god and Muhammed is his prophet?
No, you still don't get it.
The Muslim declaration of faith is: "There is no
god except
God, and Muhammad is his prophet" --- "la i
laha illal
lah, Muhammadur rasulullah".
I could become a Muslim right now by by saying (and believing) the sentence: "There is no god except God, and Muhammad is his prophet". Believing that God's
personal name is Allah would be a mistake, a blunder, a
faux pas, the amusing mistake of a beginner.
Of course we often use the word "Allah", to mean "the god of the Muslims", but that is not what it actually means in Arabic. It just means God-with-a-capital-G. If you're Jewish or Muslim or Christian or Sikh or Baha'i or Deist, then
if you're speaking Arabic, you have to say that you believe in "Allah". That is the one and only word the Arabic language has for God
qua God. That is what Arabic-speaking Christians call God. That's what Arabic-speaking Jews call God.
That's the Arabic for God.
Look at it this way. If you learned Arabic, and went to Arabia, and told a Christian Arab in Arabic that you didn't believe in Allah, then he would understand you as meaning that you were an atheist.
Do you get it now?
Edited by Dr Adequate, : No reason given.