John Gill Commentary Mark 13:21

"And then if any man shall say unto you, Lo, here is the Christ; or, Lo, there; believe [it] not:" — Mark 13:21 (ASV)

And then if any man shall say to you
During those days of tribulation and affliction, or when shortened and at an end:

lo! [here is] Christ, or lo! [he is] there ;
the Messiah has appeared in such or in such a place, to deliver you from your national distresses and calamities, and from the Roman yoke and bondage:

believe him not ;
give no credit to such a report, for the reason following; (See Gill on Matthew 24:23).