The church in Corinth had "serious" sins. Look, there were divisions, a man was sleeping with his father's wife, there was carousing, gluttony...name it! Paul's antidote to this is that "don't you know you're in Christ?" In essence, he reveals the Christ in them.
What's "strange" is that in Corinth, there was a free flow of the gifts of the Spirit! I mean look at 1 Cor 12... tongues, healing, wisdom, knowledge..name it. Why wasn't their behaviour hindering the flow of the Spirit? (???)
It's like Paul's concern was that they didn't know who they were!
But guess who Paul calls "bewitched" and "foolish". The church in Galatia. Why? For they were trying to be absolved by works of the law in this particular case - circumcision.
However, even in this church the Spirit was flowing but they seemed to be crossing over to another gospel. He asks - Gal 3:5 Then He supplying the Spirit to you and working powerful works in you, is it by works of the law, or by hearing of faith?
Both these churches had the Spirit flowing. However the common issue according to Paul is that they were departing from their roots and identity. Corinth's actions were forgetful of their identity in Christ. Galatia's faith was forgetful of the gospel of Christ.
Now for us, God have mercy. We are grappling with going back to the roots. Going back to who we are. Because if we do not know who we are in Christ, we shall be caught in the same error both these churches had, one of living according to the flesh, the other of believing according to the flesh and falling into strife, divisions and exalting everything else apart from Christ.
The root and identity is Christ. May He be formed in us! Colossians 2.