Salvation is wrought by believing.
Salvation was wrought on the cross when Christ was smitten by God, afflicted, wounded for our transgressions, bruised for our iniquities, chastised for our peace, wounded that we ourselves might be healed; when He was oppressed and sheared, cut off out of the land of the living, when the stroke was to Him, when the LORD was pleased to crushed Him and to make Him suffer.
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"and what is the surpassing greatness of His power toward us,
the believing ones, according to the working of His mighty strength which He worked in Christ in raising Him from the dead; yea, He seated Him at His right hand in the heavenlies," (Eph 1:19-20).
We are believing ones according to the working of God; as Paul goes on to say that we are saved by God's grace through faith, a gift which must be received; Christ Himself which must be received.
The non-OSAS doctrine makes
believing a work of man - how contrary to the Spirit of Grace, and to God's Gift, and the price Christ paid.
Those who endure to the end will be saved. Matthew 24:13
No doubt!
Those who believe for a while, and then depart, have become unbelievers.
An unbeliever can not become an unbeliever. However, a believer that has been born from above will always remain a believer because He has been born from above, a work of God in Christ Jesus His Son.
If someone stops believing that Jesus is the Messiah, the Lamb of God that takes away sins, then how will there sins that they commit be forgiven, seeing they no longer believe.
Believing that Jesus is the Messiah does not guarantee salvation. Many Pharisees knew beyond a shadow of a doubt that Jesus was the LORD's Christ.
Who will forgive them of their sins if they no longer believe in Jesus Christ and His work on the cross?
If only non-believers might ask this question. So, let's tell them how their sins can be forgiven!
But I guarantee this, that when you witness to a non-believer . . . you yourself would never think to tell them that their salvation can be lost by them at some point in the future. You have said as much in another post.