“Cryptography can be used to secure my data. Therefore, if I use cryptography my data is secure.”
I think Bruce Schneier described it best (paraphrased): Cryptography is like having a really strong front door on your house… 2 foot thick steal, blast proof, the whole 9 yards. A thief isn’t going to try and break through your front door… they’ll just climb through a window!
Security is about the whole system; not just the crypto. xkcd summed it up nicely: