Being justified does not mean being saved. It just means to be counted righteous. This doesn't preclude one later being declared unrighteous. However, let's consider what took place before Gen 15:6
Abrahams faith is counted as righteousness. That is believing on Him who justifies the ungodly. ( saves.)
Abraham received a sign and seal of circumcision, of the faith he had, to be the father of all them that believe, for righteousness to be imputed to them also. ( justified by faith/saved.)
The father of circumcision, to them who walk in the steps of that faith of our father Abraham, the promise was through the righteousness of faith.
God made Abraham a father of many nations, before Him whom He believed, ( not before men.) God who quickens the dead.
Abraham against hope believed in hope, ( we are saved by hope/in hope of eternal life which God who cannot lie, promised before the world began. Romans 8:24. Titus 1:2. ) to become the father of many nations, to be according to what was spoken, so shall is seed be. ( in Isaac.)
Abraham was not weak in faith, ( faith is the gift of God/the Spirit of faith, which is received through the hearing of faith. Ephesians 2:8. 2 Corinthians 4:13. Galatians 3:2) and did not consider his own body now dead, nor the deadness of Sarah's womb,
Abraham was not struggling with the promise of God through unbelief; but was strong in faith, giving glory to God; he was fully persuaded that, what God had promised, He was able also to perform.
It was imputed to him for righteousness. it was not written ( in the scriptures we have) for his sake alone, but for us also, to whom it shall be imputed, if we believe on Him that raised up Jesus our Lord from the dead; who was delivered for our offences, and was raised again for our justification. ( our justification is therefore told in James, when faith was made perfect, by the offering of Isaac.)
Romans 4:11 What shall we say then that Abraham our father, as pertaining to the flesh, hath found?
2 For if Abraham were justified by works, he hath whereof to glory; but not before God.
3 For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.
4 Now to him that worketh is the reward not reckoned of grace, but of debt.
5 But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness.
Romans 4:11 And he received the sign of circumcision, a seal of the righteousness of the faith which he had yet being uncircumcised: that he might be the father of all them that believe, though they be not circumcised; that righteousness might be imputed unto them also:
12 And the father of circumcision to them who are not of the circumcision only, but who also walk in the steps of that faith of our father Abraham, which he had being yet uncircumcised.
13 For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith.
Romans 4:17 (As it is written, I have made thee a father of many nations,) before him whom he believed, even God, who quickeneth the dead, and calleth those things which be not as though they were.
18 Who against hope believed in hope, that he might become the father of many nations, according to that which was spoken, So shall thy seed be.
19 And being not weak in faith, he considered not his own body now dead, when he was about an hundred years old, neither yet the deadness of Sarah's womb:
20 He staggered not at the promise of God through unbelief; but was strong in faith, giving glory to God;
21 And being fully persuaded that, what he had promised, he was able also to perform.
22 And therefore it was imputed to him for righteousness.
23 Now it was not written for his sake alone, that it was imputed to him;
24 But for us also, to whom it shall be imputed, if we believe on him that raised up Jesus our Lord from the dead;
25 Who was delivered for our offences, and was raised again for our justification.
James 2:21 Was not Abraham our father justified by works, when he had offered Isaac his son upon the altar?
James 2:22 Seest thou how faith wrought with his works, and by works was faith made perfect?
In Isaac shall the seed of Abraham be called. The children of the promise are counted for the seed. ( in Isaac.)
So the purpose of God according to election might stand, not of works, but of God calling.
Romans 9:7 Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.
8 That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.
9 For this is the word of promise, At this time will I come, and Sarah shall have a son.
10 And not only this; but when Rebecca also had conceived by one, even by our father Isaac;
11 (For the children being not yet born, neither having done any good or evil, that the purpose of God according to election might stand, not of works, but of him that calleth;)
12 It was said unto her, The elder shall serve the younger.
13 As it is written, Jacob have I loved, but Esau have I hated.
By faith Abraham when called, ( the purpose of God to stand, is according to election, of God calling.) obeyed.
By faith Abraham sojourned in the land of promise as in a strange country, with the heirs of the same promise, looking for a city whose maker is God. ( Holy New Jerusalem coming down from God out of Heaven. Revelation 21:2/21:10.)
Through faith Sarah received strength to conceived, she judged God faithful who had promised.
They died in faith not having received the promise, but saw them afar off, were persuaded of them, embraced them, confessing they were strangers on the earth.
Hebrews 11:8 By faith Abraham, when he was called to go out into a place which he should after receive for an inheritance, obeyed; and he went out, not knowing whither he went.
9 By faith he sojourned in the land of promise, as in a strange country, dwelling in tabernacles with Isaac and Jacob, the heirs with him of the same promise:
10 For he looked for a city which hath foundations, whose builder and maker is God.
11 Through faith also Sara herself received strength to conceive seed, and was delivered of a child when she was past age, because she judged him faithful who had promised.
12 Therefore sprang there even of one, and him as good as dead, so many as the stars of the sky in multitude, and as the sand which is by the sea shore innumerable.
13 These all died in faith, not having received the promises, but having seen them afar off, and were persuaded of them, and embraced them, and confessed that they were strangers and pilgrims on the earth.
By faith Abraham when tried offered up Isaac, Abraham had received the promises, ( of eternal life for all called to believe. God.)
It was said In Isaac his seed shall be called, Abraham accounted that God was able to raise Isaac up, even from the dead, and received him in a figure.
All received a good report through faith, but did not receive the promise, God provided something better for us, to be made perfect.
Hebrews 11:17 By faith Abraham, when he was tried, offered up Isaac: and he that had received the promises offered up his only begotten son,
18 Of whom it was said, That in Isaac shall thy seed be called:
19 Accounting that God was able to raise him up, even from the dead; from whence also he received him in a figure.
Hebrews 11:39 And these all, having obtained a good report through faith, received not the promise:
40 God having provided some better thing for us, that they without us should not be made perfect.
We see how faith wrought with the works of Abraham to make faith perfect. ( that was the accounting and receiving of Isaac in a figure, of God even raising the dead.)
Hebrews 11:40, ends on God having provided some better thing for us. Abraham had said to Isaac, God will provide a lamb for a burnt offering. ( God provided a ram, caught in a thicket by his horns, and offered him up for a burnt offering in place of his son, wen God had told Abraham to not harm his son when he was about to d as God had told him to do.)
God provided the Lamb of God, who took away the sins of the world, who was made perfect ,( they without us should not be made perfect.) became the author of eternal salvation unto all them that obey Him/ the spirits of just men made perfect.
Genesis 22:8 And Abraham said, My son, God will provide himself a lamb for a burnt offering: so they went both of them together.
Genesis 22:13 And Abraham lifted up his eyes, and looked, and behold behind him a ram caught in a thicket by his horns: and Abraham went and took the ram, and offered him up for a burnt offering in the stead of his son.
John 1:29 The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world.
James 2:22 Seest thou how faith wrought with his works, and by works was faith made perfect?
Hebrews 5:8 Though he were a Son, yet learned he obedience by the things which he suffered;
9 And being made perfect, he became the author of eternal salvation unto all them that obey him;
10 Called of God an high priest after the order of Melchisedec.
Hebrews 12:23 To the general assembly and church of the firstborn, which are written in heaven, and to God the Judge of all, and to the spirits of just men made perfect,