# 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/jit/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.