Hope this helps
1 go to settings,
2 general,
3 passcode lock,
4 (then set a passcode)
5 turn ON 'Show SMS Preview',
6 then click 'Turn Passcode Off'
7 (type in your passcode [that you just made])
that should solve your problem
BUT, if you already have a passcode set, then ignore steps 4, 6, 7