What does it mean that "My thoughts are not your thoughts" (Isaiah 55:8)?

from the old testament

• The first half of the book of Isaiah (Isaiah 1–39) focuses on God’s coming judgment against Israel, whereas the second half (40–66) focuses on God’s provided restoration.
• Historically, God’s judgment came upon Israel over a roughly 150-year period, where He used Assyria and Babylon as His instruments of judgment. As God prepared Israel for that future, He was also giving her hope that one day He would restore her and save her people. Isaiah 55:8-9 says, "For my thoughts are not your thoughts, neither are your ways my ways, declares the Lord.For as the heavens are higher than the earth, so are my ways higher than your ways and my thoughts than your thoughts."
• In the context leading up to Isaiah 55:8, God indicated that His Servant would bear the sins of His people (Isaiah 52:13–53:12) and that the result would be Zion (Israel) restored as a nation, with God as her protector (Isaiah 54).
• Isaiah 55, then, invites the reader to take advantage of these promises, with God saying, “Come, everyone who thirsts, come to the waters; and he who has no money, come, buy and eat! Come, buy wine and milk without money and without price” (v 1). His mercy is free.
• He then encourages them, saying, “Incline your ear, and come to me; hear, that your soul may live; and I will make with you an everlasting covenant, my steadfast, sure love for David” (Isaiah 55:3). Though the offer is free, it requires listening to God and turning from sin (cf. v 7b). God promises both life and everlasting love. He references David, to whom He originally gave the promise of an eternal kingdom (2 Samuel 7:12–16), showing that this promise was not new, but a further revealing of His plans.
• However, this offer will not last forever, and why He says, “Seek the LORD while he may be found; call upon him while he is near” (Isaiah 55:6). That is, “while he is near,” we have the chance to call on Him. So, it is a promise of hope, but one that must be received before the opportunity ends.
• Since the offer is limited, the wicked (contextually Israel, though ultimately all Gentiles; see Isaiah 49:6) must repent: “let the wicked forsake his way, and the unrighteous man his thoughts; let him return to the LORD, that he may have compassion on him, and to our God, for he will abundantly pardon.” (Isaiah 55:7). God’s plan of compassion is for those with a repentant heart.
• How can God both judge the wicked and forgive the repentant? Isaiah 53 provided a key hint which would be explained more fully in the New Testament, namely that there would be a suffering Servant. But because the scope of this plan exceeds humanity’s ability to comprehend, God reminds us that His thoughts and ways are far above ours (Isaiah 55:8–9).

from the new testament

• The New Testament shows how God’s plan of salvation through Jesus Christ goes far beyond human understanding. Paul emphasizes that God’s wisdom and ways surpass human reasoning, writing that “Oh, the depth of the riches and wisdom and knowledge of God! How unsearchable are his judgments and how inscrutable his ways!” (Romans 11:33). 
  
• Jesus’ death and resurrection also reveal how God can both judge sin and extend mercy: though humanity deserved punishment, God provided a way for the repentant to be forgiven (2 Corinthians 5:21).
• Salvation is shown to be a gift of grace, not something we could ever earn or fully comprehend (Ephesians 2:8–9). This is far above what we could even imagine, directly supporting Isaiah’s message that His thoughts are higher than ours.
• God’s plans are far greater than anything we could think or imagine, for His ways and thoughts are higher than ours, just as the heavens are higher than the earth. What we see as impossible, He can accomplish, and what we cannot understand, He works out perfectly according to His wisdom (Ephesians 3:20).

implications for today

How can God elect people before they believe without overriding their wills? Why does God hold everyone responsible despite His choice? Why doesn’t He just save everyone rather than requiring repentance? Christians are in never-ending discussions about how to reconcile everything Scripture teaches.

We can’t answer all our questions because God’s thoughts are not our thoughts. Imagine Moses’ wonder when God said that He is merciful and forgives yet does not leave the guilty unpunished (Exodus 34:6–7). While we cannot fully understand God and His ways, He makes Himself known to us so we can trust, respond, and marvel at His work. Truly, God's plans are far better than anything we could come up with.

God is not "like us, only bigger." He is completely unlike us-praise Him for that! He has been working out salvation since before creation. We simply cannot know the mind of God. Therefore, let us join the rest of creation and humbly marvel and wonder at how God is pulling everything together.