St. Paul said it best, "I am astonished that you are so quickly deserting him who called you in the grace of Christ and turning to a different gospel -- not that there is another gospel, but there are some who trouble you and want to pervert the gospel of Christ. But even if we, or an angel from heaven, should preach to you a gospel contrary to that which we preached to you, let him be accursed. As we have said before, so now I say again, If any one is preaching to you a gospel contrary to that which you received, let him be accursed." (Gal. 1:6-9 [RSV])
Yet again Paul says it best, "For such men are false apostles, deceitful workmen, disguising themselves as apostles of Christ. And no wonder, for even Satan disguises himself as an angel of light. So it is not strange if his servants also disguise themselves as servants of righteousness. Their end will correspond to their deeds." (2 Cor. 11:13-15 [RSV]).
John's clarification, "...This is the spirit of antichrist, of which you heard that it was coming, and now it is in the world already." (1 Jn. 4:3 [RSV])
The reason why I say Paul says it best is because if you look at the first quote I cited you can see there are people who are trying to pervert the gospel like Martin Luther saying the tradition should be thrown out the window and only the Bible should be supported even though Paul said in 2 Thes. 2:15 "So then, brethren, stand firm and hold to the traditions which you were taught by us, either by word of mouth or by letter. [RSV]" You can clearly see that Martin Luther is leading people astray. John in the third quote I cited states that the antichrist is already here and among us - if you read the Bible a little further you see that there are many antichrists. The second quote I cited, Paul shows us that Satan can even disguise himself as an angel of the Lord. That's some pretty scary stuff. But, Paul says by their fruits we shall know of them. In other words, Martin Luther who lead millions of people astray (the one who really started this whole division among Christianity [33,000 denominations in all]) is really doing the work of the devil and not of God. Satan works by putting doubt into people's minds and wants to confuse them so that they will be lost and eventually give up on God; this is how he collects his souls.