If someone does not understand eternal salvation -
how would you explain it to them in 150 words or less?
If my wife saw this, she'd laugh and tell you I've never said anything in less than 150 words in my life.
If a person denies the Biblical doctrine of eternal security, then I usually start by pointing out that, whether they realize it or not, they've adopted the Roman Catholic doctrine of infused righteousness.
I would ask them if salvation is a work of our flesh or a work God does in us and then ask them, if salvation cannot be earned, how can it be lost?
How does one become un-born again? And what happens to the new nature we receive upon regeneration if we can lose our salvation? What replaces it, since the old nature is crucified?
I would then take them to the scriptures and show them the following verses:
Romans 8:29-30 says that God predestined those whom He foreknew to be conformed to the image of Christ. If we can lose our salvation and, thus, our conformity to the image of Christ, then does this mean that God's foreknowledge is wrong? That's open theism, which has historically been condemned as heresy by the Church.
How about Jude 24, which says that Christ is able to keep us from falling? If we can lose our salvation, does this mean that Christ is able to keep us from falling, but is merely unwilling? How is that consistent with the Biblical description of Christ?
What do you make of Colosians 3:1-4, which promises that if we have been save, we will appear with Christ in glory? It doesn't say "you might appear with Christ in glory, if you don't lose your salvation". It says "you will appear with Christ in glory". Done deal.
Philipians 1:6 promises us that if Christ has begun a good work in us, that He will finish it. How does He keep that promise if you believe you can lose your salvation?
How do you explain 1 Peter 1:23, which tells us that when we are born again, we are born of incorruptable seed? If we can lose our salvation, then this seed is corruptable and that promise is not true.
Like Colosians 3:1-4, 1 John 3:2 says that we are sons of God now and that when Christ appears, we will be like Him. There is no qualifier. There is no "...if we don't lose our salvation".
Titus 1:2 says that God has promised us eternal life and that He never breaks a promise.
In John 10:27-29, Jesus promises us that He has given us eternal life, that we will never perish, and that we are not only in His hands, but in the Father's hands. How is Jesus' promise in John 10:27-28 consistent with the idea that we can lose our salvation? Is Jesus really incompetent to keep those whom the Father has given Him?
John 5:24 says that if we are saved, we will not come into condemnation but will have eternal life? How can Jesus promise that we will not come into condemnation if He knows we can lose our salvation?
Romans 11:6 tells us that salvation is not by works. If we cannot be saved by works, then how can we lose our salvation by works?
John 14:16-17 tells us that when we are saved, the Holy Spirit indwells us forever. How can the Holy Spirit indwell in us forever if we lose our salvation? Since when does the Holy Spirit dwell in the unsaved?
Ephesians 1:13, 4:30 tells us that we are sealed unto the day of redemption. If we lose our salvation, then how can we still claim to be sealed?
1 Peter 1:4 says that our salvation is "imperishable, undefiled, and unfading". If our salvation is imperishable, how can we lose it? If our salvation is promised by God to be undefiled, how can we defile it?
If we could really lose our salvation, then Hebrews 6:4-6 says that if we ever sin after being saved, we'll be lost forever with no way back, because the Lord would have to be crucified all over again to retrieve us.
Incidentally, it only takes one sin to fall away, right? I mean, one sin before we're saved was enough to condemn us, so one sin after we're saved is enough to condemn us, right? Doesn't this make the New Covenant worse than the Old? Under the Old Covenant, the Israelites were condemned for their actions, but we'd be condemned for our thoughts.
For example, they couldn't murder. We couldn't even be angry. They couldn't commit adultery. We couldn't even have a lustful thought. If the OP is right, then we lose our salvation by doing less in following the law than the Israelites did!
Is this really the Good News of Jesus Christ? Are these the riches of His Grace, that we have to live in fear of sinning? Are we saved by grace only to be placed under the constraints of an even more severely administered law?