Actually I think he pours reinforced concrete over those seeds.
Plenty of non-believers have no problem whatsoever with a "historical Jesus." It's the deity of Christ that is the problem for them. A book like this just confirms what they already think.
Often though, the seeds will not sprout until there are events in the lives of the folks in question, be it a good event or a series of events, or a bad event or a series of bad events when even the most harden non-believer to start to get the “hint”.