Exactly!
They were saved by the gospel Paul preached to them. He reminds them of that.
"I want to remind you of the gospel I preached to you, which you received, and in which you stand firm." 1 Corinthians 15:1
Then he tells them they aren't saved and their faith is in vain if they believe in a gospel of an unrisen savior. The important take away here, BB....I mean, GB...is
they were indeed saved (see above). But they aren't saved if they change the gospel Paul preached to them and by which they were saved and believe in it instead.