Isaiah relays this word of the Lord to the people: "So this is what the Sovereign Lord says: 'See, I lay a stone in Zion, a tested stone, a precious cornerstone for a sure foundation; the one who relies on it will never be stricken with panic." (Isaiah 28:16) What else (or who else, rather) could be a "tested stone" and a "sure foundation" except the Lord?
I do not believe this can be a reference to the leadership of Judah. Although King Hezekiah was on the throne during a large part of Isaiah's ministry, and although he was a king who honored the Lord and made many religious reforms in Judah, I don't believe we could call him a "tested stone". He will commit a grave spiritual error near the end of his reign, so he did not stand up to the "test" if there was one. In addition to that, I cannot think of anyplace in Scripture where a mere human being is referred to as a foundation upon which people can stand in righteousness. On the contrary, as the Apostle Paul said, there is no foundation except Christ. (1 Corinthians 3:11) Likewise, in many other passages of the Bible, it is the Lord who is being referred to as the Rock, the Solid Rock, and the Rock of Ages.
Whenever a building project was begun, the cornerstone was to laid first so the walls would be straight. This next segment is a reference to how walls were begun at the sides of the cornerstone so they would remain true. Whoever bases their life on this spiritual cornerstone---on the Lord---will be guided by the Lord's honorable and true principles. "I will make justice the measuring line and righteousness the plumb line." (Isaiah 28:17a)
We will stop at the first half of verse 17 in order to look at a previous reference to the rock that is the sure foundation. In Isaiah 8:13-14a the prophet said of the Lord, "The Lord Almighty is the one you are to regard as holy, He is the one you are to fear, He is the one you are to dread. He will be a holy place; for both Israel and Judah He will be a stone that causes people to stumble and a rock that makes them fall." I have heard this passage interpreted like this: We can stumble over the Lord (be offended by Him, reject Him) or we can fall on Him (bow to Him, submit our lives to Him, accept Him as Savior).
The Apostle Peter took our text from Isaiah 28 and applied it to the Lord Jesus Christ, quoting directly from our study material today, saying that those who trust in Him will "never be put to shame" (from today's text) and stating that those who did not want to believe in Him were stumbling over Him. (1 Peter 2:1-8) They stumbled and fell because they did not want to believe, because they found the message of the cross offensive.
The Apostle Peter also applied a similar text from Psalm 118:22 to Christ, saying, "The stone the builders rejected has become the cornerstone." The cornerstone the Lord intends to lay, here in Isaiah 28, is Christ. The Lord gave Isaiah this prophecy approximately 700 years before the birth of Jesus of Nazareth. The "builders" (the majority of Jesus' own people) rejected Him, but it is this "cornerstone" (Christ) upon which the Lord plans to base the new covenant with mankind. It is this cornerstone upon which our salvation rests. It is this cornerstone upon which the church is built. It is this cornerstone upon which the world will be governed eternally someday.
The one who trusts in Him will never be ashamed. But the one who trusts in himself, in the things of this world, or in other gods will be ashamed of those things in which they invested their faith and their time and their energy, for those things are not capable of saving them.