(4) Have ye suffered so many things in vain? if it be yet in vain.
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There were a number of accepted belief systems in the area of Palestine and the wider Roman Empire at the time this was written, such as Gnosticism and Judaism, but it is certain that God's truth was never popular or widely accepted. It is practically a foregone conclusion that someone practicing the truth will be persecuted for it to one degree or another (Matthew 13:21; Romans 8:35-36; Galatians 5:11; II Timothy 3:12; I Peter 2:19-21). In fact, the churches of Galatia (in what is now Turkey) may have been forewarned about this by Paul when he was teaching in Derbe, Lystra, Iconium, and Antioch (all on the south-eastern border of Galatia) as recorded in Acts 14:20-22. Christians are called to be separate from this world and its ways, and when the world recognizes this difference, it lashes out.
From Paul's writing, it seems that the Galatians had the proper foundation at one time, and they really did understand the truth at the beginning of their spiritual lives. This would have been the time when they were actively standing up for the truth, and a great contrast would have been evident between the Galatians and the general population. This is when they would have suffered—in the internal struggle of having to give up their former conduct, or with the external struggle of not fitting in with the rest of society.
As the Galatians began to slide into apostasy, they would no longer have been so repulsive to the people around them, and the suffering and persecution would have begun to lessen. The world would have started to recognize itself in them again (see John 15:19).
In essence, Paul is asking them if they are just going to throw away all that they had learned, especially what they had learned through adversity. With this question he is pointing out that, if they fall away, everything they had been through, both good and bad, would have been in vain in the sense that there would be no future profit from it. They would have received the maximum benefit from it already. This relates to Romans 8:28, where we are promised that all that we suffer will be redeemed for those who meet the requirements listed—those who are called according to His purpose, which the Galatian Christians were, and those who love God, which the Galatians were not doing in that they were relegating Christ's sacrifice for sin as meaningless.
— David C. Grabbe