You can't take a single verse out of context and apply it to today. God was talking to the Israelites. Look at the context:

(Deuteronomy 30:15-20 NIV) See, I set before you today life and prosperity, death and destruction. {16} For I command you today to love the LORD your God, to walk in his ways, and to keep his commands, decrees and laws; then you will live and increase, and the LORD your God will bless you in the land you are entering to possess. {17} But if your heart turns away and you are not obedient, and if you are drawn away to bow down to other gods and worship them, {18} I declare to you this day that you will certainly be destroyed. You will not live long in the land you are crossing the Jordan to enter and possess. {19} This day I call heaven and earth as witnesses against you that I have set before you life and death, blessings and curses. Now choose life, so that you and your children may live {20} and that you may love the LORD your God, listen to his voice, and hold fast to him. For the LORD is your life, and he will give you many years in the land he swore to give to your fathers, Abraham, Isaac and Jacob.

God doesn't promise us prosperity if we obey him. We're saved by the blood of Jesus. The promises God made to Israel don't apply to the church.