as of first importance

Possible meanings are 1) as the most important of many things or 2) as the first in time .

for our sins

"to pay for our sins" or "so that God could forgive our sins"

Paul is referring to the writings of the Old Testament.

This can be stated in active form. AT: "they buried him" (See: rc://en/ta/man/translate/figs-activepassive)

"Raised" here is an idiom for "caused to live again." This can be stated in active form. AT: "God raised him" or "he rose" (See: rc://en/ta/man/translate/figs-activepassive and rc://en/ta/man/translate/figs-idiom)