For I know the thoughts that I think toward you, saith the Lord, thoughts of peace, and not of evil, to give you an expected (prosperous) end.
Jeremiah 29:11 KJV
There has always been confusion on how to interpret this passage. Is this a promise made by God to everyone? Does this mean He always intends peace and prosperity for all of God's people? We will have to address then those believers who are suffering for their faith. People who are persecuted, martyred for believing the God of the Bible. Not only that, there are sincere, genuine Christians that are experiencing a variety of circumstances whether good or bad. Even the author of Ps. 73 became discouraged when he observed such phenomenon.
There are some Bible scholars that would interpret that this passage is for Israel and Israel alone. That God, in the future is thinking good things in spite of everything the Jews experienced. He will bring them back as a nation and God will rule the nations through them. This certainly seems to have some merit as Israel as of now is a strong nation in spite of its relatively small size.
And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
Galatians 3:29 KJV
For all the promises of God in him are yea, and in him Amen, unto the glory of God by us.
2 Corinthians 1:20 KJV
And if some of the branches be broken off, and thou, being a wild olive tree, were grafted in among them, and with them partakest of the root and fatness of the olive tree;
Romans 11:17 KJV
These verses gives us an explanation that its a little bit of both. This Jeremiah 29:11 is primarily for Israel but being Christians we are grafted to these promises. It is never just of physical prosperity though as we are tempted to imagine. We have to remember that the Israelites were being exiled during this time and throughout history they had horrifying experiences just being Jews. It is spiritual peace and prosperity being connected to the King of the Universe. This peace and prosperity may not be even tasted in this life but it will surely come.
And we know that all things work together for good to them that love God, to them who are the called according to his purpose. For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren.
Romans 8:28-29 KJV
This is why we need to understand message of the gospel (good news). In spite of all the pain in the world, God is doing something good, something great to show His power and His love for you. Looking back towards the history of Israel, they were conquered by the Babylonians, Medo-Persians, Greeks then the Romans yet all these nations were instrumental in bringing in the perfect circumstances for the Messiah to come at exactly the right timing. (Dan. 9:26)
If God wills, He can move the whole world to show His love for you. This is what happened in past and this is what will happen in the future. This is the future of the children of God. The important question then is if you are one of them?
But as many as received Him, to them He gave the right to become children of God, to those who believe in His name:
John 1:12 NKJV
https://bible.com/bible/114/jhn.1.12.NKJV
But now hath he obtained a more excellent ministry, by how much also he is the mediator of a better covenant, which was established upon better promises. For if that first covenant had been faultless, then should no place have been sought for the second.
Hebrews 8:6-7 KJV
https://bible.com/bible/1/heb.8.6-7.KJV