The common expression "once saved, always saved" has done the kingdom of heaven about as much good as termites have done to houses. I can speak more to Baptist circles than others, but the expression has tended to be thrown around with such casual reverence that it might as well have been found right there in the Bible.
I do find that the Bible teaches that the person who is in Christ will never find themselves outside of Christ. Thus, I would, by mere definition, agree with the terminology "once saved, always saved." However, while the term is certainly intended to grow the confidence of a believer in Jesus and to affirm something that is true, the abuse of such thinking has done considerable harm to the church, especially in the age of the invitation and the aisle walk.
The Christian 'testimony' for many has become such a direct look backward to the moment of belief that their basis for their salvation at this present moment lies more in the experience that they once knew than in their present state of knowing God through Jesus Christ.
"Holiness is the nature of the Spirit of God."
"The Spirit of God so dwells in the hearts of the saints that He there, as a seed or spring of divine nature, making the soul a partaker of God's beauty and Christ's joy, so that the saint has truly fellowship with the Father, and with His Son Jesus Christ, in thus having the communion or participation of the Holy Ghost."
As Jonathan Edwards writes in Religious Affections (page 129 in the edition by Banner of Truth Trust), the occasions for false assurance of faith are numerous. Edwards teaches that people can display spirituality and many apparent works of the movement of the Spirit of God in their lives while, in reality, they do not know true salvation in Jesus Christ. Usually such misguided people are basing their spiritual standing upon something that took place in their life that they refer to as the moment they believed.
The case can be that the person is trusting in their own conversion experience. In other words: "I believe because I came to believe." Their evidence is not found in their current life of following and obeying Christ, but in their ability to point to their own faith. Does an experience convert a person? No, believing is to hold something to be true. Biblical faith is placing trust in the gospel of God in Jesus Christ.
Are Doubts Healthy?
Edwards certainly taught that it was much better for a true follower of Christ to ask hard questions concerning his current standing in the grace of God's salvation than for the topic to be given little thought. The purpose, as Edwards states, of the detailed descriptions he gives of the evidence of faith is for the believer to be able to accurately evaluate his life Edwards, as he begins the third section of the book (page 120), states that distinguishing the true and the false in other people is not possible, but rather such examination should be a personal undertaking for one's own salvation in Christ.