This is an oft-debated question in Christian circles, but I'd like to bring up a particular situation. Rather than the case of losing salvation through a certain sin or sins, what about when a once-believer loses belief in God/ Jesus - as in - eg - Matt Dillahunty, who was devout Christian and later became an atheist?
Not only Dillahunty, but the evangelist who mentored Billy Graham when he set out to be an evangelist, Charles Templeton. He and Billy roomed together during a European crusade in 1947. He was a world class evangelist.
However, he lost his faith and concluded that the Bible wasn't true and God didn't exist, over a terrible misunderstanding of Genesis 1.
So, what does the Bible say about those who lose faith? This is what Jesus said about losing faith.
Luke 8:13 - Those on the rocky ground are the ones who receive the word with joy when they hear it, but they have no root. They believe for a while, but in the time of testing they fall away.
So Jesus acknowledges that believers can believe only "for a while", and when testing/temptations occur, "they fall away" (from their faith).
So this establishes that believers can cease to believe, which refutes the Calvinist claim that true believers will persevere in the faith. Not true, as Jesus noted.
So, what happens when a believer falls away from the faith? There are 2 verses that address this issue.
John 3:18 - Whoever believes in him is not condemned, but whoever does not believe stands condemned already because
they have not believed in the name of God’s one and only Son.
2 Thess 2:12 - and so that all will be condemned
who have not believed the truth but have delighted in wickedness.
Both verses describe those who "have not believed". It is those who "will be condemned". iow, once a person HAS believed, they shall never be condemned.
John 5:24 - “Very truly I tell you,
whoever hears my word and believes him who sent me has eternal life and
will not be judged but has crossed over from death to life.
To be condemned is to "be judged". So once a person believes Jesus says they "will not be judged", which is parallel to the 2 previous verses.
One more verse to consider:
John 10:28 - I give them eternal life, and they shall never perish; no one will snatch them out of my hand.
In this verse, Jesus said very clearly that those given eternal life "shall never perish". And He gave no exceptions to this.
So, once a person has believed in the finished work of Jesus Christ on the cross for personal sins, that person becomes a permanent child of God, possesses eternal life (John 5:24), has been regenerated (Titus 3:5), is sealed with the Holy Spirit (Eph 1:13,14). None of these things can be taken away, for any reason.
However, the Bible is also very clear about the consequences for ceasing to believe or leaving the faith. God promises discipline for His children who are rebellious or unfaithful. And the discipline is painful (Heb 12:11), and can include physical death, per 1 Cor 5:5, 11:30, Acts 5, and many other verses.
So, while there is nothing that can result in loss of salvation, every believer must be aware of the warnings about being rebellious or unfaithful to the Lord.
iow, no one will 'get away' with anything. Arminians seem to think that the doctrine of eternal security leads to sinfulness. That may occur when the doctrine of divine discipline isn't taught along side of eternal security.