I am afraid of you, lest I have bestowed upon you labour in vain.
Note 4 at Ga 4:11: There is no doubt that Paul considered these Galatians to be born-again Christians. He was the one who founded the churches of Galatia. He spoke to them as believers in this letter (Ga 1:11; 3:15; 4:12, 28, 31; 5:11, 13; and 6:1). Yet, it is also clear that Paul was in doubt about the state of their salvation (Ga 3:4; this verse, 4:20; and 5:1-4). This reveals that salvation is not an irrevocable gift (see note 1 at Mt 12:31).
Paul went on to say in Ga 5:10 that he was confident the Galatians would stand firm in the Gospel. So, in the end, Paul believed they were still standing in faith that Christ was their Savior. However, his statement here reveals that they were dangerously close to rejecting their salvation (see my note at Heb 6:4). Salvation is secure for those who keep their faith in Christ, but as we can see through the opinions expressed by Paul here, it is possible to renounce faith in Christ. Legalism taken to the extreme can lead people to becoming reprobate (see note 6 at Ro 1:28).