"I marvel that you are turning away so soon from Him who called you in the grace of Christ, to a different gospel."
Can a genuine Christian fall away from Christ? Paul's expression of amazement tells us that they can.
Apostasy can happen quickly. The Christians in Galatia had been converted to the faith for but a short time. Yet, in that short time, they were already turning away from the basics of Christianity. If anyone should have had a fresh, clear understanding of the Gospel, it was these folks. It wasn't as if long years had passed, and the fine details of their faith had gotten fuzzy in their minds. So, Paul is shocked by their quick instability.
Apostasy can be committed by an entire church. Notice that Paul addresses the entire congregation. In some of his other letters, Paul distinguishes between people. In 1st Corinthians, he learned of their church problems through Chloe's people, which implied that Chloe's people weren't part of the problem. In another letter, he implores two women named Euodia and Syntache to make up and get along, implying that the whole congregation wasn't embroiled in the strife. But in Galatians 1:4, Paul speaks to the entire group.
Apostasy is a process. Paul says that they are turning away, as if they had not completely done so yet. He is intervening, to stop the process. Doctrinal error has a way of percolating in the soul till it is fully brewed.
Apostasy is a personal break-up and a doctrinal deviation. A relationship with Jesus Christ is started and maintained through belief. Belief requires content. What would happen to my marriage if, for some bizarre reason, I began to believe that I was married to a tall, Scandinavian blond named Helga? (my wife is not tall, not Scandinavian, not blond, and not named Helga!). My marriage relationship would be destroyed by that change in my beliefs. The Gospel tells us who God and Christ are, who we are, what our need is, what Christ has done for us, and how to have a relationship with Him. Doctrinal deviation from the true Gospel and a personal break-up with Jesus are one and the same thing, essentially.
Apostasy is also a personal break-up with God, not just Jesus Christ. Paul says they are turning away from Him who called them by the grace of Christ. The "Him" is the Lord God. God the Father and God the Son are two distinct members of the Trinity, but they are not separable in a relationship sense. If you truly worship the Father, He will seek to lead you to His Son. If you fail to honor the Son, then you dishonor the Father. If you break with Jesus Christ, you have broken with God.
But apostasy can be stopped. When Paul heard about what was happening in this church, he didn't just pray. He didn't say, "Oh well, whatever God ordains to happen is what will happen." What you read here is the beginning of an intervention. Paul is going to do whatever he can to stop their turning.
This letter reminds us that real Christians have the potential to go very wrong. Our walk with God doesn't function on auto-pilot. We aren't pet dogs on a leash! Christians sometimes heed speakers they should not heed, and sometimes believe books they should not believe. Because the Bible warns us to stay on guard, I believe bad things happen to us if we don't stay on guard. I don't think the warnings in the Scripture are for hypothetical dangers that can't really happen.