That is Christians that fall into bondage after their acceptance of Christ, through continuing to live apart from conformance to the Christian life demanded by the Bible. Christians will enter into sin to address some need in their lives, then continue into a pattern of sin. Often the Christian justifies the sin as being necessary because of situations in their lives that they feel are beyond their control. Temptation is the stimulation to conform to the world's pattern of behavior instead of God's pattern. The core issue in temptation is the invitation to live independent of God and to meet needs and desires through the world, the flesh, or the devil instead of through Christ.
Temptation begins with a thought.
Most of us are familiar with the admonition in 2 Cor 10 to take captive every thought that is contrary to the will of God. However, if that vital step is not taken and the thought is entertained in our minds, the thought will lead to an action. When the action is repeated enough, it becomes a habit. As habits continue unchecked in our lives they become strongholds. The strongholds are the result of prevailing patterns of thinking and behaving.