39 For this reason they could not believe, for Isaiah said again,
40 “HE HAS BLINDED THEIR EYES AND HE HARDENED THEIR HEART, SO THAT THEY WOULD NOT SEE WITH THEIR EYES AND PERCEIVE WITH THEIR HEART, AND BE CONVERTED AND I HEAL THEM.”
41 These things Isaiah said because he saw His glory, and he spoke of Him.
42 Nevertheless many even of the rulers believed in Him, but because of the Pharisees they were not confessing Him, for fear that they would be put out of the synagogue;
43 for they loved the approval of men rather than the approval of God.” John 12:37-43.
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At first cursory thought, it is strange that God “HAS BLINDED THEIR EYES AND HE HARDENED THEIR HEART, SO THAT THEY WOULD NOT SEE WITH THEIR EYES AND PERCEIVE WITH THEIR HEART, AND BE CONVERTED”.
But, we have a clue why God did blind and harden in the following verse. “These things Isaiah said because he saw His glory, and he spoke of Him.” John 12:41.
Isaiah spoke of the glory of God, God’s commission to Isaiah to tell the people (‘Keep on listening, but do not perceive; Keep on looking, but do not understand.’ …), and God only leaving a remnant. Click here: Is6:1-ff – Passage Lookup – New American Standard Bible – BibleGateway.com However, this passage only provides a clue, because this passage does not tell us why God only left a believing remnant, other than referring that the remnant was sufficient for His purpose in Jesus (“The holy seed is its stump.”).
Nevertheless, this clue, that God did blind and harden for His glory, is sufficient, because this clue reminds us of I Corinthians 1:26-31.
“For consider your calling, brethren, that there were not many wise according to the flesh, not many mighty, not many noble; but God has chosen the foolish things of the world to shame the wise, and God has chosen the weak things of the world to shame the things which are strong, and the base things of the world and the despised God has chosen, the things that are not, so that He may nullify the things that are, so that no man may boast before God.
But by His doing you are in Christ Jesus, who became to us wisdom from God, and righteousness and sanctification, and redemption, so that, just as it is written, “LET HIM WHO BOASTS, BOAST IN THE LORD.” I Corinthians 1:26-31.
From the very beginning, God has chosen in order to get the glory.
“For you are a holy people to the LORD your God; the LORD your God has chosen you to be a people for His own possession out of all the peoples who are on the face of the earth. “The LORD did not set His love on you nor choose you because you were more in number than any of the peoples, for you were the fewest of all peoples, but because the LORD loved you and kept the oath which He swore to your forefathers, the LORD brought you out by a mighty hand and redeemed you from the house of slavery, from the hand of Pharaoh king of Egypt.” Deuteronomy 7:6-8.
“Blessed be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly places in Christ, just as He chose us in Him before the foundation of the world, that we would be holy and blameless before Him. In love He predestined us to adoption as sons through Jesus Christ to Himself, according to the kind intention of His will, to the praise of the glory of His grace, which He freely bestowed on us in the Beloved.” Ephesians 1:3-6.
1 Peter 2:9-10 ties it together for us. God has chosen His people in order to get the glory “so that you may proclaim the excellencies of Him who has called you out of darkness into His marvelous light”.
“But you are A CHOSEN RACE, A royal PRIESTHOOD, A HOLY NATION, A PEOPLE FOR God’s OWN POSSESSION, so that you may proclaim the excellencies of Him who has called you out of darkness into His marvelous light; for you once were NOT A PEOPLE, but now you are THE PEOPLE OF GOD; you had NOT RECEIVED MERCY, but now you have RECEIVED MERCY.” 1 Peter 2:9-10.
In conclusion, some state that we should hide or minimize this doctrine that salvation is unconditionally based on election, God’s choice. But, Scripture does not hide or minimize this doctrine. This “unconditional election” doctrine, the “U” of “TULIP”, the Five Points of Calvinism a/k/a Reformed Doctrine, is seen throughout Scripture to give the glory to God, “so that you may proclaim the excellencies of Him who has called you out of darkness into His marvelous light”.
Reformed Doctrine Daily Devotions 