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

"that no spirit or person easily disturb or trouble you, either by a message" 

# 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.