You're taking verses out of context. God was talking to the Israelites. Not to Christians. The Israelites were being taken as slaves to Babylon. There they would remain for 70 years. The majority would die there. The hope and the future were for the nation as a whole. God would be taking them back to their own land at the end of the 70 years.

(Jeremiah 29:4-14 NIV) This is what the LORD Almighty, the God of Israel, says to all those I carried into exile from Jerusalem to Babylon: {5} "Build houses and settle down; plant gardens and eat what they produce. {6} Marry and have sons and daughters; find wives for your sons and give your daughters in marriage, so that they too may have sons and daughters. Increase in number there; do not decrease. {7} Also, seek the peace and prosperity of the city to which I have carried you into exile. Pray to the LORD for it, because if it prospers, you too will prosper." {8} Yes, this is what the LORD Almighty, the God of Israel, says: "Do not let the prophets and diviners among you deceive you. Do not listen to the dreams you encourage them to have. {9} They are prophesying lies to you in my name. I have not sent them," declares the LORD. {10} This is what the LORD says: "When seventy years are completed for Babylon, I will come to you and fulfill my gracious promise to bring you back to this place. {11} For I know the plans I have for you," declares the LORD, "plans to prosper you and not to harm you, plans to give you hope and a future. {12} Then you will call upon me and come and pray to me, and I will listen to you. {13} You will seek me and find me when you seek me with all your heart. {14} I will be found by you," declares the LORD, "and will bring you back from captivity. I will gather you from all the nations and places where I have banished you," declares the LORD, "and will bring you back to the place from which I carried you into exile."