The definition of a Johnny-come-lately is a newcomer, or is someone who is late to accepting a position or late to arriving at an event.
A newcomer to an idea who is late in accepting what everyone else already believes to be true is an example of a Johnny-come-lately.
Informal one who is late or recent in arriving at a place, position, or viewpoint
nounpl. John·ny-come-late·lies, or John·nies-come-late·ly Informal
A newcomer or latecomer, especially a recent adherent to a cause or trend.