Paul gives his reason for writing this letter: he reminds them to continue to understand the gospel.
“I am surprised” or “I am shocked.” Paul was disappointed that they were doing this.
“Turning away” here is an idiom for “changing direction toward.” Possible meanings are 1) “you have so quickly stopped trusting in him” or 2) “you have so quickly stopped being loyal to him” (See: rc://en/ta/man/translate/figs-metaphor) (See: rc://en/ta/man/translate/figs-idiom)
“God, who called you”
Here this means God has appointed or chosen people to be his children, to serve him, and to proclaim his message of salvation through Jesus.
“because of Christ’s grace” or “because of Christ’s gracious sacrifice”
“Turning” here is an idiom for “changing direction toward.” AT: “you are instead believing a different gospel” (See: rc://en/ta/man/translate/figs-idiom).