John Piper, pastor of Bethlehem Baptist Church clearly spells out the issue. This is something I try to show to my readers through articles or blogs I post and tie it to scripture. How may Piper make such a statement?
Islam rejects that Jesus is the Son of the living God, who is God Himself, the second person of the holy Trinity, but the Bible tells us:
1 John 5:11-13 11 And the testimony is this, that God has given us eternal life, and this life is in His Son. 12 He who has the Son has the life; he who does not have the Son of God does not have the life. 13 These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life.
This is the theme of the gospels (Injil) and the entire New Testament. A faith that rejects Jesus Christ as the Biblically revealed 'lamb of God who takes away the sins of the world,' is not compatible with the Christian faith. This is a false gospel, and Islam has no guarantee of salvation. You must keep the 5 pillars of Islam, this is salvation-by-works. The Bible teaches salvation is by "grace through faith, and that not of yourselves, it is the gift of God, not of works, so that no one can boast" (Eph. 2:8,9). Anything that teaches differently, the Bible teaches is a false-gospel, leading to eternal damnation (Gal. 1:8-9).
In fact, every religion in the world teaches a form of salvation by works. It is achieved by something we can do. Christianity tells us that the heart is wicked and deceitful beyond cure, who can understand it (Jer. 17:9) and that even are good works are like filthy rags to Yahweh (Is. 64:6), so there is no hope of being saved. For since we have all sinned and fall short of the glory of God (Rom 3:23), every human on earth and througout history is justly condemned to eternal punishment in the lake of fire (Rev. 20:15), so we can only receive salvation as a gracious gift by faith in the crucifixion and resurrection of our Lord and Savior Jesus Christ.