Our Islamic friends argue that:  as the Gospel of Christ replaced the Law of Moses in the first century, so the Koran (or Quran, if you insist) replaced the New Testament as the law of God in the seventh century - and therefor the Shariah law of Islam is the law or truth of God for today.  

A dictionary will define the gospel as (1) "the teachings of Christ,” which includes the entire New Testament.  The same dictionary defines "gospel truth” (4) as "something absolutely true.”  Thus, the Gospel of Christ as revealed in the New Testament is "absolutely true.”  The modern fad of substituting "good news” for "gospel” (e.g., NIV) embraces the root of gospel but loses the  meaning of Gospel as used in the New Testament.  

The resurrection of Christ (Mark 16:15-16) was indeed "good news” to his disciples.  However, the Gospel that was preached "to every creature” (verse15) involved more than his death, burial and resurrection.  Earlier (Mark 10:28-30), when Jesus is discussing the sacrifices his disciples will face, he speaks of those who have left family "for my sake, and the gospel’s,” implying the system of faith we call Christianity as revealed in the New Testament.

According to Jude, the brother of our Lord Jesus, Christians in the first century were to "contend earnestly for the faith which was once for all delivered unto the saints” (Jude 3 ASV).  This passage indicates there will be no future revelations to replace the Faith as we have it in the revealed Word.

The permanency of the Gospel is stated by the apostle Paul in Galatians one.  Paul first points out that his apostleship was, "not of men, neither by man, but by Jesus Christ, and God the Father, who raised him from the dead” (verse 1).  He then pronounces a curse stating, "But though we, or an angel from heaven (would this not include Gabriel?), preach any other gospel unto you than that which we have preached unto you, let him be accursed” (verse 8).  He then repeats the curse  in verse nine for emphasis!

In writing to the church in Rome, Paul stresses (Romans 1:16ff) the place of the Gospel in God’s plan of salvation is:

1. The Gospel is the power of God unto salvation (Verse 16).  
2. This salvation is to those that believe the Gospel (ibid.). 
3. The "righteousness of God” is revealed in the Gospel (Verse 17).  
4. The rationale for this salvation is the Judgment (Verse 18ff).  That is, that the "wrath of
God is revealed from heaven against all ungodliness and unrighteousness of men, who
hinder the truth in unrighteousness,” ASV.

Dale I. Royal, Elk City OK