³⁸ that the word of Isaiah the prophet might be fulfilled, which he spoke: “Lord, who has believed our report? And to whom has the arm of the Lord been revealed?”

³⁸ that [purpose] the word of Isaiah the prophet might be fulfilled,

Unbelief does not deter God’s purpose. He always completes what He starts. Jewish unbelief of Jesus’ day fulfilled Scripture. The following quotes from the Old Testament prove the point. The experience of Isaiah was similar to that of Jesus.

Israel did not accept Jesus as Messiah. We can render “that” in this verse “so that.” Isaiah predicted Israel’s rejection of Jesus, and the consequence of that prediction was that Israel would indeed reject the Messiah. God did not need to do anything to cause man to sin; his proclivity to sin was already inveterate, entrenched, and chronic in his soul.

The purpose clause shows that God was not frustrated by Israel’s rejection of Jesus. Our will cannot disturb God’s plan. Israel’s hardening was the means whereby God carried out His plan; their rejection of Jesus led to His dying on the cross for our sins. Both the cross and the crown of God were the outcome of this plan.

which he spoke:

Here is a quote from Isaiah 53:1 (LXX). The forecast of John’s gospel by this quote is that Israel would ultimately reject Jesus as their Messiah. He was deemed “despised and rejected of men.”

“Lord, who has believed our report?

Isaiah prophesied that Israel would not believe in the Messiah. They did not believe the extant words of Scripture given by Isaiah. Rejection of the Word of God was nothing new to Israel. Israel of Jesus’ day refused His message just like they had with Isaiah. They were responsible to believe but they did not accept the offered Messiah.

And to whom has the arm of the Lord been revealed?”

Only a few would believe when the Messiah comes (Isa 53:1). The “arm of the Lord” is figurative language for God’s sovereignty. God demonstrated His power in the signs, or miracles, of Jesus. People cannot come to salvation by operation bootstraps; God must take the initiative by drawing them to Himself. This involves His sovereign placing people, places, sickness, blessing, and many other sovereign actions into the path of people to lead them to a point of decision.

PRINCIPLE:

Hardening of unbelief entrenches God’s will against those who refuse to believe.

APPLICATION:

God’s order in creation demands that those who willfully harden themselves against His will He will harden further. The issue is how we use our will. Both the human being and God Himself are involved in the hardening when we violate God’s plan for how we use our will.

This happened to Pharaoh. The more he hardened himself against God, the more God hardened his heart. We never turn negative against God without paying a price. God hardened Pharaoh’s heart, but it was after multiple chances to exercise positive volition. His persistent and insistent rejection of God’s plan put him in a place of a hardened heart.

God always hardens further those who have a proclivity to resist His Word. This is His divine order in executing divine concursus. Guilt always remains with those who use their volition improperly (Ro 9:17). God’s actions toward Pharaoh hardened his heart even further (Ex 8:32; 9:12). The hardening of the heart toward God has its consequences. Rejection of God’s actions did not frustrate His purpose.

John’s point in the quotation of Isaiah 53:1 was not that God manufactured their disbelief. No one can believe without God first drawing him or her to Himself. Yet there will always be those who fail to believe in God’s powerful revelations of Himself. The combination of faith and divine interactions is an operating principle throughout Scripture.

Note the specificity in Jesus’ fulfilment of the Isaiah 53 prophecy. Despite the miraculous signs Jesus presented to Israel, still they would not believe on Him.

For a detailed look at concursus, go here: https://versebyversecommentary.com/articles/doctrine/concursus/gods-decree-and-free-will/