You can find out who Tom Harpur is somewhere else.
But I'll paraphrase one of Tom Harpur's critical points; you decide the merits of his argument for yourself.
"Look at all the things recorded in the gospels following Christ's crucifixion that are never mentioned in secular sources from the same period.
"Like, the temple curtain was allegedly ripped open. A solar eclipse took place. There was an earthquake that opened tombs, and dead people were walking around. Those should all be obvious things, yet somehow none of that is recorded or mentioned during those times anywhere outside of the gospels."
Ok, so the Isis/Horace myth allegedly pre-dates Christ's resurrection by 2500 years. That should be an obvious thing, yet somehow none of those well-known 2500-year old "similarities to Christ" were recorded or mentioned during that period by the Romans or the temple leaders, who woulda loved any excuse to discredit the thousands of pesky new Christians running around the countryside proclaiming "HE IS RISEN."
Just saying ... and neither could the Romans or the temple priests produce Christ's body to contradict the gospel accounts.