Here is this passage from the ESV:
As you come to him, a living stone rejected by men but in the sight of God chosen and precious, you yourselves like living stones are being built up as a spiritual house, to be a holy priesthood, to offer spiritual sacrifices acceptable to God through Jesus Christ. For it stands in Scripture: “Behold, I am laying in Zion a stone, a cornerstone chosen and precious, and whoever believes in him will not be put to shame.” So the honor is for you who believe, but for those who do not believe, “The stone that the builders rejected has become the cornerstone,” and “A stone of stumbling, and a rock of offense.” They stumble because they disobey the word, as they were destined to do. (1 Peter 2:4-8 ESV)
We as Christians are "stones" in the building that is the Church. We see in our world every day how these "living stones" (Christians) are being rejected by men. We see it in the laws that are passed. We see it when activists sue Christian bakery and flower shop owners for exercising their consciences. We see it in the massacres and beheadings of Christian families in Iraq and elsewhere. Christians are rejected.
We may be rejected by men, but God chose us and saved us and considers us precious. He has chosen us to be a holy priesthood to offer acceptable spiritual sacrifices through Christ. What is a spiritual sacrifice?
I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service. -Romans 12:1
And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God. -Romans 12:2
Jesus is the corner stone. If our trust is in Him we will not be ashamed. But to those who do not believe on Him He is a stumbling stone and is offensive to them. They stumble because they disobey the word, as they were destined to do. Here is a question: were they destined to disobey the word and stumble or were they destined to stumble as a result of their disobedience?
I think it works both ways.
What shall we say then? That the Gentiles, which followed not after righteousness, have attained to righteousness, even the righteousness which is of faith. But Israel, which followed after the law of righteousness, hath not attained to the law of righteousness. Wherefore? Because they sought it not by faith, but as it were by the works of the law. For they stumbled at that stumblingstone; As it is written, Behold, I lay in Sion a stumblingstone and rock of offence: and whosoever believeth on him shall not be ashamed. (Romans 9:30-33 KJV)
They were destined to stumble because they disobeyed in trying to attain righteousness by works.
And not only this; but when Rebecca also had conceived by one, even by our father Isaac; (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;)
So then it is not of him that willeth, nor of him that runneth, but of God that sheweth mercy. For the scripture saith unto Pharaoh, Even for this same purpose have I raised thee up, that I might shew my power in thee, and that my name might be declared throughout all the earth. Therefore hath he mercy on whom he will have mercy, and whom he will he hardeneth. Thou wilt say then unto me, Why doth he yet find fault? For who hath resisted his will? Nay but, O man, who art thou that repliest against God? Shall the thing formed say to him that formed it, Why hast thou made me thus? Hath not the potter power over the clay, of the same lump to make one vessel unto honour, and another unto dishonour? What if God, willing to shew his wrath, and to make his power known, endured with much longsuffering the vessels of wrath fitted to destruction: And that he might make known the riches of his glory on the vessels of mercy, which he had afore prepared unto glory, (Romans 9:10-11, 16-23 KJV)
This passage shows they disobey and stumble because God hardened their hearts.
Man's nature is fallen. All men in their fallen natures have a free will to act within that fallen nature. Man will only act within his fallen nature and reject God, unless God changes his heart and gives him a desire to come to Him in faith and obedience to Him. Is God unjust to give mercy and faith to some and not others? Nay but, O man, who art thou that repliest against God? Shall the thing formed say to him that formed it, Why hast thou made me thus? Hath not the potter power over the clay, of the same lump to make one vessel unto honour, and another unto dishonour?
Let's say you have one thousand criminals on death row and the President chooses to give a Presidential pardon to fifteen of them. Is he being unjust to pardon the fifteen. No. Would he be unjust to not pardon any? No. They were all destined to die anyway. He just chose to show mercy on some. Is God unjust to show mercy on some sinners but not on others? All have sinned and are headed for Hell. Is He obligated to our standard of justice (fairness)?
My take home lesson in all of this is
1. I am precious to God no matter what others may say.
2. Am I offering spiritual sacrifices acceptable to God through Jesus Christ?
3. Do I show by my life that Jesus Christ is precious to me?
4. God is sovereign over all things...even salvation.