en_tn_lite_do_not_use/1co/15/03.md

as of first importance

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

for our sins

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

according to the scriptures

"just as the propets predicted in the scriptures." Here "scriptures" means the Old Testament.