«Christians and Muslims believe in the same God, the one God»

Along the path marked out by Abraham in his submission to the divine will, we find his descendant, the Blessed Virgin Mary, Mother of Jesus, who is also devoutly invoked by Muslims, especially in popular piety.
We Christians joyfully recognize the religious values we have in common with Islam. Today I would like to repeat what I said to young Muslims some years ago in Casablanca:
"We believe in the same God, the one God, the living God, the God who created the world and brings his creatures to their perfection",
VIII/2, [1985], p. 497).

See: Muslims and Christians adore the one God

Christian Response
The Christian response to Muslims should be twofold. Firstly, we must separate ourselves from Islam and clearly state that it is a false religion. 'Whoever transgresses and does not abide in the doctrine of Christ does not have God. He who abides in the doctrine of Christ has both the Father and the Son. If anyone comes to you and does not bring this doctrine, do not receive him into your house nor greet him; for he who greets him shares in his evil deeds' (2 John 9-11). Christians do not have any ecumenical relationship with Muslims. We cannot participate in their idolatry by saying that we worship the same God. On the contrary, we must warn them that since they do not abide in the doctrine of Christ, they do not have God.
Secondly, we have an evangelistic responsibility towards Muslims. They have been indoctrinated against the Son of God. We must proclaim Jesus, the Son of God, the Lord from Heaven, the Saviour of the World. We must proclaim that he died for our sins according to the Scriptures, that he was buried, and that he rose again the third day according to the Scriptures. This is our message to Muslims, and to the rest of the world, 'He who believes in the Son has everlasting life; and he who does not believe the Son shall not see life, but the wrath of God abides on him' (John 3:36).
Read more: Do Catholics and Muslims Worship the Same God?