If not by force, then by subtilty.

"I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel."

Note the resourcefulness of the devil. Heretics do not advertise their errors. Murderers, adulterers, thieves disguise themselves. So the devil masquerades all his devices and activities. He puts on white to make himself look like an angel of light. He is astoundingly clever to sell his patent poison for the Gospel of Christ. Knowing Satan's guile, Paul sardonically calls the doctrine of the false apostles "another gospel," as if he would say, "You Galatians have now another gospel, while my Gospel is no longer esteemed by you."

We infer from this that the false apostles had depreciated the Gospel of Paul among the Galatians on the plea that it was incomplete. Their objection to Paul's Gospel is identical to that recorded in the fifteenth chapter of the Book of Acts to the effect that it was not enough for the Galatians to believe in Christ, or to be baptized, but that it was needful to circumcise them, and to command them to keep the law of Moses, for "except ye be circumcised after the manner of Moses, ye cannot be saved." As though Christ were a workman who had begun a building and left it for Moses to finish.

Today the Anabaptists and others, finding it difficult to condemn us, accuse us Lutherans of timidity in professing the whole truth. They grant that we have laid the foundation in Christ, but claim that we have failed to go through with the building. In this way these perverse fanatics parade their cursed doctrine as the Word of God, and, flying the flag of God's name, they deceive many. The devil knows better than to appear ugly and black. He prefers to carry on his nefarious activities in the name of God. Hence the German proverb: "All mischief begins in the name of God."

When the devil sees that he cannot hurt the cause of the Gospel by destructive methods, he does it under the guise of correcting and advancing the cause of the Gospel. He would like best of all to persecute us with fire and sword, but this method has availed him little because through the blood of martyrs the church has been watered. Unable to prevail by force, he engages wicked and ungodly teachers who at first make common cause with us, then claim that they are particularly called to teach the hidden mysteries of the Scriptures to superimpose upon the first principles of Christian doctrine that we teach. This sort of thing brings the Gospel into trouble. May we all cling to the Word of Christ against the wiles of the devil, "for we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places."

~Dr. Martin Luther, Commentary on Galatians 1:6.