The closest local airport is Lehigh Valley International (ABE) – http://www.flylvia.com/ – which has connections to many major hubs. Be advised that if your flight itinerary includes a “connection” to or from Newark airport, this is not a flight, but a bus service. Connections from the Philadelphia airport are indeed flights.
Lehigh is 65 miles from Philadelphia International Airport (PHL) and 75 miles from Newark Liberty International Airport (EWR). From Newark, the Trans-Bridge bus service can transport you directly to south Bethlehem, less than a ½ mile from Lehigh’s campus: http://www.transbridgelines.com/newark-airport-schedule.html.
There is no organized transportation from the Philadelphia airport, but Uber will service passengers from PHL to Bethlehem.
Other New York area airports (La Guardia, JFK) are reasonably close, and Trans-Bridge runs bus service from JFK to South Bethlehem as well: http://www.transbridgelines.com/jfk-airport-schedule.html.