Growing up I was taught that once you were saved, you were saved forever and could not lose your salvation, but over the years I’ve found too many verses that don’t seem to support this view.
I agree a lot of verses do talk about pretenders, but there are a few more that seem to talk about people rejecting the Spirit after having received it. Here is an example from Luke 8:
He spoke by way of a parable: “The sower went out to sow his seed; and as he sowed, some fell beside the road, and it was trampled under foot and the birds of the air ate it up. Other seed fell on rocky soil, and as soon as it grew up, it withered away, because it had no moisture. Other seed fell among the thorns; and the thorns grew up with it and choked it out. Other seed fell into the good soil, and grew up, and produced a crop a hundred times as great.” As He said these things, He would call out, “He who has ears to hear, let him hear.”
His disciples began questioning Him as to what this parable meant. And He said, “To you it has been granted to know the mysteries of the kingdom of God, but to the rest it is in parables, so that ‘seeing they may not see, and hearing they may not understand’.
“Now the parable is this: the seed is the word of God. Those beside the road are those who have heard; then the devil comes and takes away the word from their heart, so that they will not believe and be saved. Those on the rocky soil are those who, when they hear, receive the word with joy; and these have no firm root; they believe for a while, and in time of temptation fall away. The seed which fell among the thorns, these are the ones who have heard, and as they go on their way they are choked with worries and riches and pleasures of this life, and bring no fruit to maturity. But the seed in the good soil, these are the ones who have heard the word in an honest and good heart, and hold it fast, and bear fruit with perseverance.
Remember, when Jesus is talking about the Word, he does not mean scripture. He is talking about the active, immediate prompting of the Holy Spirit–the voice of God. He is talking about himself. I think this is also what Hebrews 3 is talking about.
I understand this is a hard truth. But God seeks us wholly. He asks us for our love. We can be angry, sad, frustrated, confused, and everything else, but at the end of the day there must be commitment from our end.
Hebrews 3 twice says to hold fast until the end. And as Jesus says, we must hold him fast and bear fruit with perseverance.