William Guthrie, in his The Christian’s Great Interest, shows how the Bible is clear that a person can know he is saved. Nevertheless there remains still the phenomemon of widespread uncertainty amongst people. Guthrie gives five reasons why this is the case:
- Ignorance of God and his ways.
- Dealing deceitfully with God and their own consciences.
- Slothfulness and negligence.
- Having no fixed idea of what evidence would satisfy them.
- Their dependence on changeable evidence.
The first three seem obvious and eye-opening to me. (Isn’t that often the case? The obvious thing is the one we are most likely to neglect!)
The final point is the problem of looking inwardly to my own diligence in spiritual disciplines. Guthrie lists: success in defeating sin, a ‘successful’ prayer life, the inner witness of the Spirit as the evidences that are changeable. Clearly, we keep sinning, our prayer life can be perplexing and the inner witness can be hard to detect. But none of these experiences have any bearing on the fact of our salvation.
Doggie's Breakfast 
Stephen,
Great post. One of the greatest treasures that I found when coming to the doctrines of grace and a better understanding of the Scriptures was a better grasp of justification. Many people today often focus on their “success” in the process of sanctification (our progress in the faith) with their justification (our standing with God). However, when they realize that BOTH things are totally and ultimately in the hands of God and His Holy Spirit, I think that some assurance does arise from that.