The part that demons play in either reducing our assurance or increasing our complacency
Often the doubt and fear about whether we are saved is caused, or intensified, by demons whispering into a real Christian’s mind. They do it deliberately because they want to create anxiety and uncertainty. Alternatively, if you are already leaning towards being complacent, the demon will want to increase your complacency and get you to be even more casual. Thus the demon will do the exact opposite. They will try to convince false Christians/unbelievers that they are saved, whereas they try to get real Christians to doubt their salvation.
They will always try to get you to lean farther in whatever direction you are already leaning. So, if you are an anxious type to begin with, they will get you to be even more anxious. If however you are already a complacent type, they will seek to make you even more complacent.
This deceptive activity by demons is a major problem in both directions. See Books Seven and Nine in this series for more detail on demons and what they do. Are you in the sincere, earnest, tender-hearted group? Do you find you are demoralised by doubts about whether you are really a Christian? Do you feel useless, unworthy and, "too sinful for God to forgive" etc? If so, then take heart. Such thoughts are not your own. Neither are they from God.
They are just lies being planted in your mind by a demon. Their aim is to discourage and weaken you by robbing you of your assurance. The demon will whisper those unsettling thoughts to you precisely because he knows you are a tender, sensitive type and that you are likely to accept such lies and be made even more anxious.
God never speaks in those destructive ways or says such negative things. If God wants to tell you that you aren’t a real Christian He does it by ‘convicting’ you, not by condemning or accusing you. There’s a vital difference. Conviction is healthy and leads you to repent. Condemnation is unhealthy and leads you to despair. So, ask God to guide you as to whether what you are feeling or hearing is from Him, or from a demon.
If your thoughts about yourself make you feel generally useless, condemned and worthless, then you can be sure it is based on demonic whisperings. If so, reject it all and press ahead in your Christian life with even more determination and zeal. If you do that then the demon's trick will have backfired on him. He will have overdone it, as they often do.
If however what you are feeling is a very specific conviction that you need to repent and stop doing a particular sin, then you can be sure that that is God's voice. He never tells you that you are a worthless failure. He says things like: "What you just said was a lie. Put it right immediately and don't lie again".
That's the wholesome and constructive way in which the Holy Spirit speaks. It is totally unlike what the demons say about you being a worthless, no good failure etc. The Holy Spirit would never use destructive words like that. Remember that fact next time you feel bombarded by such condemning or disparaging thoughts. The tone and content of the words will tell you where they are coming from. Then you will be better able to ignore what the demons say and hold on to what God is saying.