Here’s how I would apply that approach in the case of Jeremiah 29:11. In Jeremiah 29:11, we learn that God cares for His people and doesn’t abandon them, and that He hasn’t abandoned them even if it seems everything has gone wrong in their lives. We see that His good purposes are still moving forward, even when everything looks grim. This, we can cling to, because it’s an understanding of God’s character, which doesn’t change. What we can’t cling to is the specific outworking of God’s character in that unique biblical situation.
The most basic place to start is to look at what is said to New Covenant believers versus those who were under the specific terms of the Mosaic Covenant with its promised blessings and curses. If a promise was made as a term of the Mosaic Covenant (for example, that they would have good crops if they obeyed the Covenant’s commands), then that is not part of our Covenant. However, we can learn much about who God is from how He dealt with people in the Mosaic Covenant, and His character does not change. From His dealings with people in the Old Testament, we learn that He is just, that He is gracious, that He loves undeservedly, that He’s trustworthy and faithful to those He’s covenanted with, that He has their ultimate good in mind, that He’s working everything together for His plan to glorify Himself and redeem the world. God doesn’t change, so what we learn about Him in the Old Testament is still true of Him today. The only question is about how He promises to act today on those characteristics of love, grace, etc.
When we get to the New Covenant, we see these same characteristics of God play out in terms of the New Covenant. For example, “because of His great love with which He loved us,” and by His grace, we’re reconciled to God through Christ’s death on the cross and are made alive with Him; we are given the Holy Spirit as a secure pledge of God’s faithful covenant with us (Ephesians 1–2). (All who are united to Christ are also heirs of the promises made by grace to Abraham—Galatians 3:15–29.) The particular promises and commands of this Covenant may be different, but His character is the same. We know specifically that His love, grace, wisdom, and purpose are directed towards making His people like Christ (Romans 8:29), and that everything in our lives is working towards that end good (Romans 8:28). It’s the trustworthy character of God that we cling to throughout this difficult life, regardless of whether He heals us or increases our “crops.”
So when reading the Old Testament, we look to see what we can learn about God in any given passage. Who is He? What is He like? How does He treat His people? In terms of promises, the biggest promise of the Bible is the promise of who God is. This is the promise we depend on. We find we have cancer? We trust the promise that God is good, that He’s sovereign over history, that He loves us, and that He seeks the good of His people. We can’t trust that He’ll heal us (that isn’t always the greatest good for us and for God’s kingdom), but we can trust that God is good and sovereign, and that our suffering has a purpose.
Always look for what you can learn about God through His dealings with human beings throughout the Bible.