en_tn_lite_do_not_use/2th/02/02.md

that you not be easily shaken from your thoughts

This metaphor speaks of people who are frightened or uncertain as if a person had moved what they had been standing on so that they fell down. Alternate translation: "that you not easily let people disturb you" (See: rc://en/ta/man/translate/figs-metaphor)

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.