it was just as it is written

Here Paul refers to a scripture where Christ (the Messiah) speaks to God. You can translate this in an active form. Alternate translation: "the Messiah said to God in the scriptures" (See: rc://en/ta/man/translate/figs-explicit and rc://en/ta/man/translate/figs-activepassive)

The insults of those who insulted you fell on me

The insults of those who insulted God fell on Christ.