Adultery is sin and sin can be forgiven and cleansed through the blood of Jesus Christ.
The scripture says, Rom 3:23 For all have sinned, and come short of the glory of God; BUT, BUT, BUT
Rom 3:24 Being justified freely by His grace through the redemption that is in Christ Jesus:
Rom 3:25 Whom God hath set forth to be a propitiation through faith in His blood, to declare His righteousness for the remission of sins that are past, through the forbearance of God;
Rom 3:26 To declare, I say, at this time His righteousness: that He might be just, and the Justifier of him which believes in Jesus.
1Jn 1:6 If we say that we have fellowship with Him, and walk in darkness, we lie, and do not the truth:
1Jn 1:7 But if we walk in the light, as He is in the light, we have fellowship one with another, and the blood of Jesus Christ His Son cleanses us from ALL sin.
1Jn 1:8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
1Jn 1:9 If we confess our sins, He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
1Jn 1:10 If we say that we have not sinned, we make Him a liar, and His word is not in us.
So WE have no place to judge others. Muhammad was not God's judge. Even Jesus did not judge others. God is the righteous judge.
God's Righteous Judgment
Rom 2:1 Therefore thou art inexcusable, O man, whosoever thou art that judgest: for wherein thou judgest another, thou condemnest thyself; for thou that judgest doest the same things.
Rom 2:2 But we are sure that the judgment of God is according to truth against them which commit such things.
Rom 2:3 And thinkest thou this, O man, that judgest them which do such things, and doest the same, that thou shalt escape the judgment of God?
Rom 2:4 Or despisest thou the riches of His goodness and forbearance and longsuffering; not knowing that the goodness of God leadeth thee to repentance?
God's love and goodness leads men to confess their sins and repent of them, not so He can have them stoned, but so that He can forgive them their sin and cleanse them from ALL unrighteousness through the blood of Jesus! God didn't send Jesus to CONDEMN the world but that through Him they might have eternal life! That is God's righteous judgement. It is all through Jesus Christ. Jesus is the Way, Truth and the Life and NO MAN comes to the Father without Him.