Genesis 2:16-17, "He said, 'Of all the trees in the garden, thou mayest freely eat, but of the tree of knowledge of good and evil, thou mayest not eat. For on the day you eat of it, thereof, ye shall surely die"
Unfortunately, Adam and Eve, through their free choice did eat of the forbidden tree and got lost eventually (Genesis 3:6-7). The LORD God did pronounced judgment on Adam and Eve, but at the same time promised a Redeemer who will one day break the serpent's head. Even though Adam and Eve are in their state of being saved, they chose not to be saved since they, by their free will, ate the forbidden fruit. So, based on this, ONCE SAVED ALWAYS SAVED is not applicable for them.
Next, when God chose the people of Israel as His own people with Moses and Aaron as leaders, they are in a saved condition. But eventually, they worshiped the golden calf and not the God of Israel, who commanded, "Thou shalt not make strange gods before Me" (Exodus 20:3), and therefore in the end, did not enter the rest that God promised them (Psalm 95:9-11).
Afterwards, came the kings of Israel such as Saul and David. Saul once was obedient to the LORD's commands and one time disobeyed the LORD by not waiting for Samuel to offer the sacrifice before the battle with Philistia. However, Saul deliberately disobeyed the LORD by sparing the animals and cattle of the Amalekites, to the point that he was rejected and consulted the witch of Endor instead of consulting the LORD and ended up dead in the battlefield at Gilboa (1 Samuel 15, 31, 1 Chronicles 10:13-14).
"Obedience is better than sacrifice and submission than the fat of rams" (1 Samuel 15:22)
Then afterwards came David and Solomon. David was a humble shepherd from Bethlehem who won victory after victory with the LORD's help. He was crowned king who unified Israel and Judah and made Jerusalem the capital city of Israel. He was, "a man after God's own heart" (Acts 13:22) but disobeyed God by taking Bathsheba and killing Uriah by putting him in the front line of the battlefield where he is to be killed (2 Samuel 11:14-15). The LORD punished David but eventually repented and his saved condition was restored.
John 6:64, "'Yet some of you do not believe'. Jesus knew from the very beginning who were the ones who did not believe, and which one will betray Him"
NOTE: Judas wasn't among those, "who did not believe", he was among, "the one who will betray Him". In fact, Judas still had the privilege of being an Apostle like he is able to cast out demons and heal and preach in Jesus' Name (Mark 6:7-13).
However, Judas deliberately betrayed Jesus to the Pharisees who promised Him thirty silver pieces (Matthew 26:14-16) and killed himself, thus being damned (Matthew 27:3-5).
Based on these examples of people from the Bible, what can we conclude about Eternal Security?
1. Eternal Security is not Biblical. There are tons of Biblical evidences wherein a person who is once saved can fall away from the faith like in Romans 11:21-22 and here's what it says:
Romans 11:21-22, "God did not spare the natural branches, do you think He will spare you? Here we see the kindness and severity of God, severity to those who are fallen, but to you, kindness if you will continue in His kindness. Otherwise, you too will be cut off"
and even the book of Hebrews speak strongly on apostasy:
Hebrews 6:4-6, "For it is impossible for those who were once enlightened, who had a share of the Holy Ghost, who have tasted the good word of God and the powers of the coming ages, if they shall fall away to renew them again to repentance, seeing to themselves that they are crucifying the Son of God afresh and exposing Him to public shame"
2. Eternal Security enables Christians to lower their guard.
The Apostles often warn us that the devil is like a prowling lion, waiting for someone to devour (1 Peter 5:8-9), which is why they said this:
1 Corinthians 10:12, "If anyone thinks he is standing firm, let him take heed lest he fall"
3. Eternal Security promotes a "security in sin" doctrine.
Some Fundamentalists are teaching that a person can still sin and stay saved at the same time. But that is not what our Lord teaches. He even said in John 5:14 and here's what it says:
John 5:14, "Afterwards, Jesus found Him in the Temple and said, 'Listen! You are well now! So stop sinning or something worst may happen to you'"
Jesus Himself warned to that person who had been cured after 38 years based on John 5:5 to stop sinning or something worst may happen to him.The Apostle Peter even compared those people who fell away to a dog and a pig as he said in 2 Peter 2:20-22
2 Peter 2:20-22, "If they have escaped the pollution of this world through the knowledge of our Lord and Savior, Jesus Christ and then again caught up by them, the former is worst in the end than it was at the beginning. Better for them to have not known the way of righteousness than to know it and turn away from the sacred commandment given to them. They are only doing what the proverbs said to be true, 'A dog goes back to his own vomit' and, 'A pig that is washed is back to roll in the mud'"
The wages of sin is death (Romans 6:23), even for an unfaithful child of God. That is why, we need to live holy lives for without which, no one will see the Lord (Hebrews 12:14) and James emphasized that it is also by works we are justified and not by our faith alone (James 2:24).
To close this up, I will quote Matthew 7:21, Matthew 10:22 and John 14:15
Matthew 7:21, "Not everyone who says to Me, 'Lord, Lord', will enter the Kingdom of heaven, but only those who do the will of My Father who is in heaven"
Matthew 10:22, "Everyone will hate you because of Me, but whoever endures to the end will be saved"
John 14:15, "If you love Me, you will obey My commandments"
"For the time will come when people will not accept sound doctrine, but will hire for themselves teachers to suit their itching ears. They will cover their ears from the truth and give attention to legends (2 Timothy 4:3-4)