Once Saved Always Saved is biblical. Eternal Security is a biblical doctrine. Now let me guess, are you one of these people that believe that you can be saved and then be lost, and then be saved again and then be lost again and on and on? I have never heard a preacher who believed in eternal security say that once a Christian is saved that he can live anyway he wants to live and be as wicked as he chooses to be. It is religious folks who like to cause a new born again to doubt his eternal security by telling him that he can lose his salvation. I am going to show you from the Holy Scriptures that a Christian in this dispensation known as the Church Age cannot lose his salvation. Here are the Scriptures:
He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him. - John 3:36
And this is the record, that God hath given to us eternal life, and this life is in his Son - 1 John 5:11
These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God - 1 John 5:11
33 Who shall lay any thing to the charge of God's elect? It is God that justifieth. 34 Who is he that condemneth? It is Christ that died, yea rather, that is risen again, who is even at the right hand of God, who also maketh intercession for us.
35 Who shall separate us from the love of Christ? shall tribulation, or distress, or persecution, or famine, or nakedness, or peril, or sword?
36 As it is written, For thy sake we are killed all the day long; we are accounted as sheep for the slaughter.
37 Nay, in all these things we are more than conquerors through him that loved us.
38 For I am persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come,
39 Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord. - Romans 8:33-39
And this is the Father's will which hath sent me, that of all which he hath given me I should lose nothing, but should raise it up again at the last day. - John 6:39
That the saying might be fulfilled, which he spake, Of them which thou gavest me have I lost none. - John 18:9
In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise, - Ephesians 1:13
And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption. - Ephesians 4:30
Verily, verily, I say unto you, He that believeth on me hath everlasting life. - John 6:47
When you get saved, the Holy Spirit baptizes you into Christ's body. Therefore our salvation is not ours to lose. Remember that salvation is a gift. And also remember that God is not an Indian giver. Once you are born again, you cannot be unborn again.
Also notice that both Ephesians 1:13 and Ephesians 4:30 says that we are sealed by the Holy Spirit. And as Ephesians 4:30 says, we are sealed unto the day of redemption.
Now, if a Christian does sin, even though his sins have already been forgiven, he still will have to reap the consequences.
Be not deceived; God is not mocked: for whatsoever a man soweth, that shall he also reap. - Galatians 6:7
If a Christian becomes so carnal that he commits fornication after fornication and so on, he will not lose his salvation. Now while he cannot lose his salvation, he can still lose his testimony, his joy, his fellowship with God the Father, his health, his money, and even his life. But not only that, this Christian man can also lose his eternal rewards at the Judgement Seat of Christ and his millennial inheritance. So, while a Christian man or woman in this dispensation known as the Church Age cannot lose their salvation, they can still lose a lot of other things.
Oh and by the way, that verse that you took from Matthew.
But he that shall endure unto the end, the same shall be saved. - Matthew 24:13
That verse is talking about the Tribulation also known as the Time of Jacobs Trouble. And no Christian will be here for that. Since it is not the Church's trouble, but rather the time of Jacob's trouble.
Many pastors who attack the doctrine of Eternal Security usually are men who have not learned how to rightly divide their Bible. There are divisions in the Bible. There are dispensations in the Bible, and when you rightly divide them, there is no confusion. The confusion comes, when people try to take a verse from one dispensation and apply it doctrinally to another dispensation.