My 2 cents -
Islam is by definition an antichrist religion and driven by the antichrist spirit. Just because someone calls themselves Christian does not mean that they are one. Many lawless deeds have been done in the name of Christianity..
I John 4:1-6 -
1Beloved, do not believe every spirit, but test the spirits, whether they are of God; because many false prophets have gone out into the world. 2By this you know the Spirit of God: Every spirit that confesses that Jesus Christ has come in the flesh is of God, 3and every spirit that does not confess that Jesus Christ has come in the flesh is not of God. And this is the spirit of the Antichrist, which you have heard was coming, and is now already in the world. 4You are of God, little children, and have overcome them, because He who is in you is greater than he who is in the world. 5They are of the world. Therefore they speak as of the world, and the world hears them. 6We are of God. He who knows God hears us; he who is not of God does not hear us. By this we know the spirit of truth and the spirit of error.
The Holy Bible, New King James Version, (Nashville, Tennessee: Thomas Nelson, Inc.) 1982.
Matthew 7:21-23 -
21Not everyone who says to Me, Lord, Lord shall enter the kingdom of heaven, but he who does the will of My Father in heaven. 22Many will say to Me in that day, Lord, Lord, have we not prophesied in Your name, cast out demons in Your name, and done many wonders in Your name? 23And then I will declare to them, I never knew you; depart from Me, you who practice lawlessness!
The Holy Bible, New King James Version, (Nashville, Tennessee: Thomas Nelson, Inc.) 1982.
We are called to preach the gospel to everyone. Muslims are no exception, (Mark 16:15). There are probably many Christians who currently call themselves Muslim out of ignorance. To echo what Melanie said, if we do not show them Jesus, then they will not see Jesus. When true Christians make them thirsty for the water of the Word of God because of their saltiness, (Matt. 5:13), then they will convert. Otherwise, God will send an angel fom heaven to teach them directly, (Rev. 14:6-7).
Be blessed,
Dr. Ray