Christians were calling God Illah (God, cf. El, as in Ezekiel, Daniel, pl. Elohim) or Allah (the God, cf. the God of Abraham, the God of salvation etc) before there were Muslims or a Koran, let alone before Arabic spread through the world as the sacral and scriptural language or Islam.
In the Melkite Divine Liturgy, the Trisagion begins “Kuduson il’lah” – “Holy God”.