en_tn/2th/02/02.md

that you not be easily disturbed or troubled, either by a spirit or by a message

This can be translated in active form. Alternate translation: "that no spirit or person easily disturb or trouble you, either by a message" (See: rc://en/ta/man/jit/figs-activepassive)

by a message, or by a letter that seems to be coming from us

"by spoken word or by written letter that pretends to be coming from us"

to the effect that

"saying that"

the day of the Lord

This refers to the time when Jesus will come back to the earth for all believers.