I believe that the best explanation of this topic can be found in Luke chapter 8 known as the parable of the sower:
Luke 8 (Amplified Bible) -
4 And when a very great throng was gathering together and people from town after town kept coming to Jesus, He said in a parable:
5 A sower went out to sow seed; and as he sowed, some fell along the traveled path and was trodden underfoot, and the birds of the air ate it up.
6And some [seed] fell on the rock, and as soon as it sprouted, it withered away because it had no moisture.
7And other [seed] fell in the midst of the thorns, and the thorns grew up with it and choked it [off].
8And some seed fell into good soil, and grew up and yielded a crop a hundred times [as great]. As He said these things, He called out, He who has ears to hear, let him be listening and let him [
b]consider and understand by hearing!
9And when His disciples asked Him the meaning of this parable,
10He said to them, To you it has been given to [come progressively to] know (to recognize and understand more strongly and clearly) the mysteries and secrets of the kingdom of God, but for others they are in parables, so that, [though] looking, they may not see; and hearing, they may not comprehend.
11Now the meaning of the parable is this: The seed is the Word of God.
12Those along the traveled road are the people who have heard; then the devil comes and carries away the message out of their hearts, that they may not believe ([
c]acknowledge Me as their Savior and devote themselves to Me) and be saved [here and hereafter].
13And those upon the rock [are the people] who, when they hear [the Word], receive and welcome it with joy; but these have no root. They believe for a while, and in time of trial and temptation fall away (withdraw and stand aloof).
14And as for what fell among the thorns, these are [the people] who hear, but as they go on their way they are choked and suffocated with the anxieties and cares and riches and pleasures of life, and their fruit does not ripen (come to maturity and perfection).
15But as for that [seed] in the good soil, these are [the people] who, hearing the Word, hold it fast in a just ([
d]noble, virtuous) and worthy heart, and steadily bring forth fruit with patience.
16No one after he has lighted a lamp covers it with a vessel or puts it under a [dining table] couch; but he puts it on a lampstand, that those who come in may see the light.
17For there is nothing hidden that shall not be disclosed, nor anything secret that shall not be known and come out into the open.
18Be careful therefore how you listen. For to him who has [spiritual knowledge] will more be given; and from him who does not have [spiritual knowledge], even what he thinks and [
e]guesses and [
f]supposes that he has will be taken away.
I believe that the scripture is clear, some will come to the faith yet will fall away either due to self centeredness, self preservation or simply bad doctrine. I believe that is why proper discipleship is so important that believers might become mature in their faith. Some who hold to certain doctrinal traditions might say that these people were never saved to begin with. To me it really doesn't matter if you believe that they have willingly forfeited their salvation or they were never saved to begin with, the truth is that people do walk away.