Ephesians 1:13 In him you also, when you had heard the word of truth, the gospel of your salvation, and had believed in him, were marked with the seal of the promised Holy Spirit; (NRSV - Catholic Ed.)
and again...
Ephesians 4:30 And do not grieve the Holy Spirit of God, with which you were marked with a seal for the day of redemption. (NRSV - Catholic Ed.)
The believer is sealed in the Holy Spirit which cannot be broken, only by the one intended to open the sealed gift, which is Christ. So, it there any way one can lose their salvation? Jesus says this:
Matthew 12:31- 32 Therefore I tell you, people will be forgiven for every sin and blasphemy, but blasphemy against the Spirit will not be forgiven. Whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come. (NRSV - Catholic Ed.)
And how does one blaspheme against the Holy Spirit? It isn’t clearly explained but I believe it is to grieve the Spirit in some excessive way such as ascribing some evil to the Spirit or renouncing one’s salvation or perhaps committing suicide.
I don’t believe it can be lost for most common sins (I hate to categorize them as common because all sin is against God) but what good would it do for salvation to be turned on and off like a light switch based on your last thought or action? It would make any honest person incredibly anxious knowing their salvation can be easily lost with a momentary slip.
As I have aged, I believe all the more that our God sees us as the vulnerable short-sighted children that we are and that He has taken it upon Himself to see that our salvation is secure and the only way it can be lost is by some deliberate action of defiant rejection of God’s free gift.
A requirement of confession and absolution for every common transgression would lose every soul that died suddenly by a bullet, a plane crash, a heart attack or car accident that had no time to seek a priest before they expired.
Perhaps this explains why so many ex-Catholics, tired of living under constant eternal uncertainty, seek relief through a church that believes God knows our lives from beginning to end and forgives generously, with rare exception, to all those who earnestly seek Him.
Psalm 103:8-14 The Lord is merciful and gracious, slow to anger and abounding in steadfast love. He will not always chide, nor will he keep his anger forever.
He does not deal with us according to our sins, nor repay us according to our iniquities. For as high as the heavens are above the earth, so great is his steadfast love toward those who fear him; as far as the east is from the west, so far does he remove our transgressions from us.
As a father shows compassion to his children, so the Lord shows compassion to those who fear him. For he knows our frame; he remembers that we are dust.