"I am astonished that you are so quickly deserting him who called you in the grace of Christ to another gospel" (Galatians 1:6)
The word in the above passage translated as "deserting" is the Greek word metatithemi. It literally means to change from position. In this passage, Paul is saying that he is astonished that the Galatians are so quickly changing from their position in Christ to a different gospel.
Another important word here is the word "quickly." Notice what this word implies. We must ask, Quickly after what?" The word implies that these Galatians are changing their position soon after some event. Soon after what? Well, if we look into the letter we see Paul remembering that they had "began in the Spirit" and had "received the Spirit" (3:3,5) and they had been "running well" (5:7) after he had preached the gospel to them. Obviously here, we have a case of people who were born again but were turning from Christ to a different gospel and changing their position from Christ.
Paul says later of these same individuals that if they return to the Law, then Christ died for nothing (2:21), they are "ending in the flesh" having began in the Spirit (3:3), Paul feels now that he has preached to them in vain (4:11) if they continue in this vein, Christ will have profited them nothing (5:2), and "You have been voided from Christ... [and] have fallen from grace" (5:4) and because these individuals were ending in the flesh Paul says, "I warn you... those who live like this will never enter the Kingdom of God" (5:21).
Obviously, Paul is indicating these people have turned away from the position they enjoyed in Christ and were now following a different gospel. They were people who were born again into the new life of the Spirit, but were returning to the old way of the flesh under the Law through the rite of circumcision in the flesh. As such, Paul sees they have forsaken Christ and if they persist in their ways, they will not enter the Kingdom of God.
"You have fallen from grace" (Galatians 5:4).