Satan. If you remember the parable of the sower and the seed there were two groups that started well. There were those who received the good word and started on the journey and then could not finish and there were those who endured to the end. The thing is that those who will not be decieved by the devil God will protect from his deception because they have a sincere love for the truth. There are some good christians who do not have a sincere love for the truth and these shall fall away. But is never right to say that someone who fell away was never even saved before as most OSAS people say.
If salvation is the result of a changed heart or new birth, who caused the changed heart and new birth? Will the one Who gave the new birth undo it?
Remember that Paul says that in the latter time many shall depart from the faith giving heed to seducing spirits and doctrines of devils.
Paul, inspired by God, also said, "Being confident of this very thing, that he which hath begun begun a good work in you will perform it until the day of Jesus Christ." (Pilippians 1:6)
Were not these people once in the faith?
"They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us." (1 John 2:19)
Wouldn't that suggest they were saved and on their way to the kingdom. But they departed and allowed the devil to seduce them into the world. The answer to your first question is here. The devil's target is not those in the world, for he has them already. It is those in the church and he will get some to fall away, to depart from the faith. That is why the bible says that he that endures to the end will be saved since not all will endure.
Those who have the new birth are the ones who endure to the end. It isn't their works that keep in them in the faith. Their faith from and in God keeps them so that what they do for God will endure.
"For whatsoever is born of God overcometh the world: and this is the victory that overcometh the world, even our faith."
(1 John 5:4)
Where does faith come from? It comes from God, the Giver of all good.
1And you hath he quickened, who were dead in trespasses and sins; 2Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience: 3Among whom also we all had our conversation in times past in the lusts of our flesh, fulfilling the desires of the flesh and of the mind; and were by nature the children of wrath, even as others. 4But God, who is rich in mercy, for his great love wherewith he loved us, 5Even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved; ) 6And hath raised us up together, and made us sit together in heavenly places in Christ Jesus: 7That in the ages to come he might shew the exceeding riches of his grace in his kindness toward us through Christ Jesus. 8For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: 9Not of works, lest any man should boast. 10For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them.
(Ephesians 2:1-10)
We are God's workmanship, not our own.
9For I am the least of the apostles, that am not meet to be called an apostle, because I persecuted the church of God. 10But by the grace of God I am what I am: and his grace which was bestowed upon me was not in vain; but I laboured more abundantly than they all: yet not I, but the grace of God which was with me.
(1 Corinthians 15)
Were we not dead in our sins until Christ quickened us?
17And if Christ be not raised, your faith is vain; ye are yet in your sins.
(1 Corinthians 15)
20But now is Christ risen from the dead, and become the firstfruits of them that slept. 21For since by man came death, by man came also the resurrection of the dead. 22For as in Adam all die, even so in Christ shall all be made alive.
(1 Corinthians 15)
5Even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved; )
(Ephesians 2)
We can't take credit for what God has clearly done in our hearts.
Respectfully,
Phoebe Ann