Only God infallibly knows the hearts of people and who truly are and are not saved. Judas Iscariot looked like the real deal to the 11 remaining disciples but Jesus knew his heart and said, "he is a devil!" (John 6:70-71) The Lord loves justice and does not forsake His saints; they are preserved forever.. (Psalm 37:28)
In regard to the parable of the sower, in Luke 8:13, even though this shallow ground hearer is said to have "believed," he is never said to have been "saved." How do we know that the shallow ground hearer was never actually "saved?" I will give the reasons. First, his heart condition is contrasted with that of the "good ground" hearer, who's heart was "good" and "honest." Thus, his heart was not "good," being like the soil to which it corresponds, being "shallow" or "rocky," lacking sufficient depth. Such soil represents a sinner not properly prepared in heart.
People who "believe" and "rejoice" at the preaching of the gospel without a prepared heart, and without a good and honest heart, and without having "root" in themselves, do not experience real salvation. Unlike saving belief, temporary shallow belief is not rooted in a regenerate heart. How can no depth of earth, no root, no moisture, no fruit, represent saving belief? It can't and it doesn't. Faith without works is dead.
IN CONTRAST TO - Mark 4:8 - But other seed fell on good ground and yielded a crop that sprang up, increased and produced: some thirtyfold, some sixty, and some a hundred. Luke 8:15 says, But the ones that fell on the good ground are those who, having heard the word with a noble and good heart, keep it and bear fruit with patience. The rocky soil represents a person not properly prepared in heart so the seed planted ends up with a lack of "root" (lack of being firmly rooted and established).
The same Greek word for "believe" (pisteuo) is used in James 2:19, in which we read that the demons "believe" (pisteuo) "mental assent" that "there is one God," but they do not believe in the Lord Jesus Christ (Acts 16:31) and are not saved.
John has portrayed people who "believe" to some level or degree but are clearly not saved. There is a stage in the progress of belief in Jesus that falls short of firmly rooted and established belief resulting in salvation. As we see in John 2:23-25, in which their "belief" was superficial in nature and Jesus would not entrust/commit Himself to them.
Also, in John 8:31-59, where the Jews who were said to have "believed in him" turn out to be slaves to sin, indifferent to the words of Jesus’, children of the devil, liars, accused Jesus of having a demon and were guilty of setting out to stone and kill the one they have professed to believe in.
So there is no loss of genuine faith or salvation here and tose who are sealed with the Holy Spirit of promise are sealed until the redemption of the purchased possession. (Ephesians 1:13-14) Praise the Lord! :)
This is not about over thinking it. It's about properly harmonizing scripture with scripture before reaching our conclusion on doctrine which avoids contradictions in scripture.