The enemy can actually use either of these three against you: Sin, Your weakness or Your ignorance.  People are mostly aware of the sin aspect but ignorant and unaware of some other devices of the enemy such as the two listed. 

Sin: This is plainly disobedience to God's word and His commandments. Demons gladly infiltrate this lifestyle. People open up themselves to the demonic and to curses as mentioned in the bible. (Deuteronomy 28:13-68)

Your weakness: This may not necessarily be sin. For example poverty, inability to speak or do a particular thing or just simply something ones not so good at; lack of exercise; unhealthy diet; lack of sleep;  choosing bad company or association etc. 

Your ignorance: This is when you are clueless on spiritual matters. Sadly the kingdom of darkness does not care if one is ignorant but in fact they like people that that way. However the bible has let us know that the truth we know sets us free (John 8:31-32). Therefore we need to seek truth. 

The enemy will attack and infiltrate your life if you are in any or all of these types of situations. He also preys upon generational curses. This will be discussed at another time