what is the earliest recorded use of the word Allah?

 The earliest recorded use of the word Allah is not precisely known, but it appears as early as the first century. An inscription using the Ancient South Arabian script in Old Arabic from Qaryat al-Fāw reads, "'to Kahl and lh and ʿAththar' (b-khl w-lh w-ʿṯr)"

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. The word Allah has been used by Arabic people of different religions since pre-Islamic times. According to some scholars, the Kaaba was first consecrated to a supreme deity named Allah and then hosted the pantheon of Quraysh after their conquest of Mecca, about a century before the time of Muhammad

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. Some inscriptions seem to indicate the use of Allah as a name of a polytheist deity centuries earlier, but nothing precise is known about this use

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The word Allah is used by Arabic speakers of all Abrahamic faiths (including Christianity and Judaism) as meaning "God"

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. The name's origin can be traced to the earliest Semitic writings in which the word for god was il, el, or eloah, the latter two used in the Hebrew Bible (Old Testament) 

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how did the use of the word Allah change after the advent of Islam?   Before the advent of Islam, the word Allah was used by Arabic people of different religions to refer to the supreme deity. The pre-Islamic Arabs worshipped Allah alongside other lesser deities. According to some scholars, the Kaaba was first consecrated to a supreme deity named Allah and then hosted the pantheon of Quraysh after their conquest of Mecca, about a century before the time of Muhammad. 

After the advent of Islam, the use of the word Allah became more specific to refer to the Islamic conception of God. Muslims believe that Allah is God's proper name, while Christians and Jews know Him as YHWH or Yahweh. The word Allah is still used by Arabic speakers of all Abrahamic faiths as meaning "God." When Arabic-speaking Christians use the word Allah, it is usually used in combination with the word al-Ab, meaning "God the Father," to distinguish themselves from Muslims.