Won't start

Two main reasons, if battery is checked and has enough cranking power your alternator is most likely the problem. If your Anti-theft light flashes that means your key is not registering with your cars computer and it usually engages the kill switch where you should just hear a "clink" if you hear same clinking sound repetitively then you have an electrical issue. Your issue seems more related with your key, if you only have one ask your Ford dealership to check. Also keep in mind if you leave the key in ACC state for about a minute it usually helps the computer establish a connection with your keys magnet.