The Greek word used is ἀνόητος (anóētos), which comes from a (without) + noiéō, (to think). Therefore, this refers to not thinking, not reasoning through a matter, being unmindful, or acting in a mindless, dense way. This is rendered in English as “stupid.”
There are three reasons that he calls some people stupid:
1. They have looked away from the Cross. The text says,
O stupid Galatians! Who has bewitched you, before whose eyes Jesus Christ was publicly portrayed as crucified? (Gal 3:1)
Paul engages in a kind of wordplay here. The Greek word translated here as bewitched is βασκαίνω (baskaino), which more literally means to give an evil eye to someone. Hence, they have permitted someone to give them an evil eye even though Christ had been portrayed to their eyes as crucified.
For us, the danger is that we look away from the Cross of Jesus and think that there is some other way we can be saved. Some evil eye looks at us or winks at us to draw our attention away from Christ. The cross is an absurdity to the world, but to us who are being saved it is the power and wisdom of God. Yet, we are too easily convinced to look away to passing pleasures or sinful sights. We adopt the bewitched notion that these can give us the happiness we seek.
St. Paul calls this stupid, and deep down we know that this is so. No limited thing can fill the God-sized hole in our heart. Only through the Cross of Christ can we attain to that place where true joys are found. Only the Cross can purify us and get us to Heaven, where God and God alone can show us joys unspeakable and glories untold.
An eye that looks anywhere else is the evil eye of a stupid person.
2. They think they can save themselves. The text says,
I want to learn only this from you: did you receive the Spirit from works of the law, or from faith in what you heard? Are you so stupid? (Gal 3:2-3)
Thinking we can save ourselves is a stupid though. I can barely perform minor surgery on myself, let alone save myself. Do you have a ladder tall enough to reach Heaven? Do you have a rocket ship powerful enough to soar to Heaven? And even if you did, how would you know the coordinates of Heaven so that you could soar there? Stupid, stupid, stupid!
No, the only way we can get to Heaven is to listen to God by faith and follow Him in the grace of that faith working through love (see Gal 5:6). We often think that we will be well through science, medicine, therapy, wealth, popularity, and so forth. This is stupid. Individually and collectively we have attained to all these things and yet I would argue that we are more miserable than ever. We have never been healthier and yet we have never worried more about our health. We have never been more well fed and yet we have never worried more about what we eat. Despite this, we easily think these sorts of things will fill the bill. St. Paul simply calls this “stupid.”
3. They focus on the flesh not the spirit. The text says,
Are you so stupid? After beginning with the Spirit, are you now ending with the flesh? (Gal 3:3)
We are too easily preoccupied with our bodies and with things related to the flesh and the material world. That’s pretty stupid, really. Such things are dying, rusting, and decaying. In contrast, the things of the Spirit will last forever. A bowl of porridge is just a meal, but the Word of the Lord endures forever; it can be a saving word for us and the joy of our soul even now. We must care for the body, but our obsession with looking good and our pathological fear of the bodily effects of aging manifest a stupid and foolish preoccupation. The soul should be our priority, for our good treatment of it will have eternal effects. Instead, we focus on what passes and ignore what is eternal. Paul has a word for this: stupid.
Don’t be stupid.