You need to do a wipe to restore firmware.
Completely powered OFF.
Pull the battery, if necessary, to make sure it's off.
Reinstall the battery, but don't turn the phone on.
Press and HOLD volume UP, home and power at the same time.
Hold all three until you see the word Samsung, then release all three buttons immediately.
If you release a little too soon or too late, the phone will boot normally, so try again.
Now it should load to Recovery Mode and you can recover.