en_tn_lite_do_not_use/2th/02/02.md

that you not be easily disturbed or troubled

"that you do not easily let yourselves be disturbed"

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.