Satan is too clever and too powerful for us to handle by ourselves. But the One who has conquered him and disarmed him of his powers is our Saviour (Col.2:15). Our Lord is able to keep us from falling (Jude.24), either to the lusts in our flesh or to the schemes of the Devil. But it is dangerous to presume on this and walk about carelessly because the wily Devil is moving around looking for naive Christians who can be trapped (1Pe.5:8). If we want to be safe, we need to put on the whole armour of God (Ep.6:11,12).
If we allow any chink in our armour, the Devil is going to attack us through that gap. This is what we need to be careful about, to keep the whole armour on.
God warns us not to allow anger to persist in our heart for long, because the Devil will use that to work up his attack against us by pumping angry thoughts and revengeful ideas into us till we end up in violent action or breaking up relationships (Ep.4:26,27). If we hold on to unforgiving attitudes towards anyone else, that gives the Devil another foothold in our life to work with (2Co.2:10,11). These two passages illustrate the point that if we hold on to any sin knowingly in our life we have a chink in our armour. Then we can't bank on God to keep us from falling. Jesus told us to watch and pray knowing our weakness (Mt.26:41).
There are people who dabble with occult practices out of curiosity or for thrills. These are direct contacts with demonic powers, and if we play with fire we can't hope to come away unharmed. Many have opened the way for demons into their life through this path. Another entry for demons is through worship of idols because the power behind lifeless idols is Satan working through his demons (1Co.10:20).
God tells us not to serve our bosses with 'eyeservice' (Ep.6:5,6), doing our work well only when they are watching us. When we, as Christians, are expected to walk before God, do we do right and avoid what is wrong only when people are watching us? If so, there are bound to be many chinks in our armour! Then it will be no wonder if the Devil is able to attack us, in our thoughts first and then increasingly more powerfully in other areas too. This can happen even if we don't believe in demons or say that we are in Christ and expect the blood of Christ to protect us! It will be we ourselves who open parts of our armour for the devil to attack us.
The answer is not just in getting 'delivered', because if the chinks in the armour are not closed up, the attacks will resume! There has to come a repentance in our life and a beginning of fear and trembling in working out our salvation. We should not take grace for granted or expect Jesus to give us protection while we continue with sin or wrongdoing. God has accepted us out of His unmerited favour towards us, and now He gives us grace (power) to walk in His ways. But if we choose to continue in sin or to enjoy its pleasures, we will suffer the consequences (Ga:6:7,8).