Well, St. Paul set the standard. “If anyone comes to with a gospel other than the one I preach, even an angel” etc. So let’s see if someone comes up to you with golden tablets and wings and subtle (or radical) changes to the gospel and its theology/christology, you’re still supposed to say, “uh no thanks.”
That said, the church fathers raised the bar a little higher. Even translators of the bible who twist the meanings from the original traditions(i.e gnostics, et. al.) are anathema (c.f. Tertullian)
Pretty clearly, therefore, Mohammed, Charles Taze Russell, L. Ron Hubbard, and other “new religion” founders would be “not Christian.”
Sorry Mr. Card.
If you honestly study Gal 1 you will see Paul talked about his time as he conversed with the Lord who altered Paul that his words were being changed as they spoke and than Paul said that he received the Lord's Gospel directly.
Gal 1
6 I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:
7 Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
9 As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed.
10 For do I now persuade men, or God? or do I seek to please men? for if I yet pleased men, I should not be the servant of Christ.
11 But I certify you, brethren, that the gospel which was preached of me is not after man.
12 For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ
The Lord is speaking currently not in the future but Paul's present.
So, according to you, Paul wrote the Revelation of St. John the Divine?
Preach it, brother. My thoughts exactly.