Before we answer this question, we need to consider exactly what the word “Amen” means. The first mention of the word Amen in the Bible is found in Numbers 5:22 which says, “Then the woman shall say, ‘Amen, amen’” (KJV). In some translations it reads, “Then the woman shall say, ‘Amen, so be it’ (NKJV; NIV). In this passage (verses 16-22) the priest is putting the woman under an oath and if she were to be found guilty of unfaithfulness to her husband she would be cursed. In saying, “Amen, so be it” she would be “agreeing to it”; in other words, she would agree that she deserved to be cursed if found guilty.

In the New Testament the word has the same meaning. It can mean “surely,” “truly,” or “so be it” and thus the person saying “Amen” is “agreeing with the truth of what was just said.” For example, in Philippians 4:20 the Apostle Paul, as inspired by the Holy Spirit (see 2nd Peter 1:21), said “Now to our God and Father be glory forever and ever. Amen.” Surely every believer agrees that God deserves all the glory for all eternity for all that He has done for us and when they hear someone praise God with these words, they would give a hearty “Amen” to those words. Paul ended that letter in verse 23 with these words, “The grace of our Lord Jesus Christ be with you all. Amen.” Again, every believer knows we need the “grace of our Lord Jesus Christ” every minute of every day and would gladly respond by saying, “Amen!” The last two verses in our Bible read, “He who testifies to these things says, ‘Surely I am coming quickly. Amen. Even so, come, Lord Jesus! The grace of our Lord Jesus Christ be with you all. Amen” (Revelation 22:20-21). What believer would “disagree with this?” Every believer longs for the Lord to come and would gladly respond to this promise with that blessed word “Amen,” followed by “Even so, come, Lord Jesus.” And every believer feels the need for His grace to sustain us until He comes and so their joyful response to the Apostle John’s wish for His grace to be with us is “Amen.”

Now let’s answer your question. Is it correct to say Amen to the prayer of a Muslim faithful? We need to ask, “What would the prayer of a Muslim faithful be?” In other words, what does a faithful Muslim believe about God and about Jesus Christ? Do they believe in the true God and Jesus Christ? Listen to what their so-called Bible (the Koran) says in Sura 4, Ayat 171: “O People of the Scripture, do not commit excess in your religion or say about Allah except the truth. The Messiah, Jesus, the son of Mary, WAS BUT A MESSENGER OF ALLAH…so believe in Allah and His messengers.” The Christian’s Bible never refers to God as “Allah.” In Ephesians 1:3 we read, “Blessed be the God and FATHER or our Lord Jesus Christ.” Why is He called “Father?” Ah, because He has an eternal Son, who was sent to this earth to become a Man in order to be the “Savior of the world.” This is brought out in 1st John 4:14, “And we have seen and testify that the Father sent the Son to be the Savior of the world” (KJV). That the Son is God who became a Man is brought out in John 1:1 & 14, “In the beginning was the Word, and the Word was with God, and the Word was God…And the Word was made flesh, and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, full of grace and truth” (NKJV). We saw that in the Muslim scriptures Jesus is “ONLY A MESSENGER of Allah among other messengers.” To them Jesus is NOT GOD; He is NOT THE SON OF GOD; and He did NOT BECOME A MAN in order “to be the Savior of the world.” So, the “faithful Muslim” denies the Person and the Work of Christ and thus he would never utter a prayer about God and His Son that the true believer in Christ could respond to with a hearty “Amen.”

In closing, we read in 2nd John 9-11, “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.” This passage would surely apply to a Muslim or anyone else who does not believe in the true doctrine of Christ, and we learn here that we should not invite them into our house if they come knocking on our door with their false doctrine. If we do, we are in essence, agreeing with their doctrine and having fellowship with them. They are actually “enemies of Christ” and if we invite them into our house, we are guilty of taking sides with them against Christ. This most definitely means we should not let them pray with us either. If we were in another location and we heard “a prayer of a faithful Muslim,” we should not agree with them by saying “Amen.” Having said that, we should still LOVE THEM and PRAY FOR THEM (for their SALVATION), and if given the opportunity (at some other time when they are not coming to us with their Muslim doctrines) we should PRESENT THE GOSPEL TO THEM (see 1st Timothy 2:3-6). I should mention that I canvass communities “door to door with gospel literature” and I have met Muslims and have presented to them the gospel. In those instances, I am coming to them with the “doctrine of Christ” and I have liberty to engage them, though I would still not “pray with them” if they were to ask me if they could “pray for me.”  (DO)  (525.1)