The phrase "the last hour" refers to the time just before Jesus returns to the earth and judges all people. This can be translated as "Jesus will return soon." (See: [[rc://en/ta/man/translate/figs-metonymy]])
## by which we know ##
This can be translated as "and because of this we know" or "and because many antichrists have come, we know."
## many antichrists have come ##
"there are many people who are against Christ."
## They went out from us ##
"They left us"
## but they were not of us ##
"But they did not really belong to us anyway" or "they were not really part of our group in the first place." The reason they were not really part of the group is that they were not believers in Jesus.
## For if they had been of us, they would have continued with us ##
This could be translated as "Because, if they were really believers they would not have left us."
## they went out in order that they might be shown that they are not of us ##
This can be stated with an active clause: "they left us so that God could show us they were never really believers." (See: [[rc://en/ta/man/translate/figs-activepassive]])