Here "firstfruits" is a metaphor, comparing Christ to the first of the harvest, which would be followed by the rest of the harvest. Christ was the first to be raised from the dead. Alternate translation: "who is like the first part of the harvest" (See: [[rc://en/ta/man/translate/figs-metaphor]])
"Raised" here is an idiom for "caused to live again." This can be stated in active form. Alternate translation: "God has raised Christ, who is the firstfruits of those who died" (See: [[rc://en/ta/man/translate/figs-activepassive]] and [[rc://en/ta/man/translate/figs-idiom]])