True Believers for whom Christ died are preserved and safe forever because they are Christs Seed whom God promised they shall endure forever. Isa 53:10
10 Yet it pleased the Lord to bruise him; he hath put him to grief: when thou shalt make his soul an offering for sin, he shall see his seed, he shall prolong his days, and the pleasure of the Lord shall prosper in his hand.
Now Christ is the antitypical David whom God made a Covenant with, and Christ being born of the seed of David Rom 1:3
3 Concerning his Son Jesus Christ our Lord, which was made of the seed of David according to the flesh;
God made Covenant with Him Ps 89:3-4
3 I have made a covenant with my chosen, I have sworn unto David[Christ] my servant,
4 Thy seed will I establish for ever, and build up thy throne to all generations. Selah.
Now it goes on to say, in regards to Davids Seed or Christs Seed Isa 53:10 this:
Ps 89:27-36
27 Also I will make him my firstborn, higher than the kings of the earth.
28 My mercy will I keep for him for evermore, and my covenant shall stand fast with him.
29 His seed also will I make to endure for ever, and his throne as the days of heaven.
30 If his children forsake my law, and walk not in my judgments;
31 If they break my statutes, and keep not my commandments;
32 Then will I visit their transgression with the rod, and their iniquity with stripes.
33 Nevertheless my lovingkindness will I not utterly take from him, nor suffer my faithfulness to fail.
34 My covenant will I not break, nor alter the thing that is gone out of my lips.
35 Once have I sworn by my holiness that I will not lie unto David.
36 His seed shall endure for ever, and his throne as the sun before me.
Christs seed will, shall break, transgress Gods laws, for they are sinners, however God in mercy will chastise them, but will never suffer His faithfulness and mercy towards them to fil, and will cause them to endure forever, even as Christs Throne.
So the believers eternal security is foundational in Gods Covenant Faithfulness to His Son. God has sworn their eternal security. Compare Heb 6:13-18
3 For when God made promise to Abraham, because he could swear by no greater, he sware by himself,
14 Saying, Surely blessing I will bless thee, and multiplying I will multiply thee.
15 And so, after he had patiently endured, he obtained the promise.
16 For men verily swear by the greater: and an oath for confirmation is to them an end of all strife.
17 Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath:
18 That by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us:
God takes His swearing upon Himself very serious and that's where their eternal security lies!
