Paul expects to see a tenacity in true believers which makes us proof against false teaching. We have had the witness of God in our hearts that these things are true. When God first calls us to himself, he gives us a body of knowledge which he assures us by his Spirit is true, and it agrees with what is in the word of God. This anointing is sufficient to guard us against any perversion of the gospel. There is therefore no reason why even the youngest believer should fall away.
The gospel is inviolable. It is of overriding importance, and is the one thing that must be honoured even at the expense of an apostle. What God says is to be believed even though all mankind or even all the angelic host should contradict it. This is a duty of man, and if it had been followed would have delivered him from falling when faced with the lies of the serpent in the Garden of Eden who was indeed an angelic being.
God may allow severe tests of his people to try their faith, even to the point of appearing to go against himself (Exodus 32:10), but all the time he desires that we hold on to what has been revealed. His people must hold on to the truth of Scripture in spite of persecution, the fall of respected leaders, the denial of the faith of those who originally lead them to Christ, miraculous or supernatural evidence to the contrary. But the Galatians had fallen to a far lesser test than this. They had fallen to the teaching of the Judaizers, while Paul had remained faithful, so that they were without excuse. Let us therefore be encouraged by his example, not that we lean on Paul, but that we have confidence in the God who can put constancy in our hearts also.
Finally, these two verses teach what a fearful condemnation will swallow up false teachers. For if even Paul the apostle and a chosen instrument of God, or a good angel must suffer the curse of God for teaching error, how much more would the false teachers suffer condemnation?