Why Does God Promise Safety in the Old Testament but Difficulty in the New Testament?

Q: In the Old Testament, it seems that God promises safety to the faithful, but in the New Testament, He seems to promise difficulty. Why is that?

A: Part of the promises of God for safety, blessing, or good in the Old Testament were in the context of the covenant promises that God made to ancient Israel, centered at the Sinai covenant, the old covenant, the Mosaic covenant. In the covenant God made with Israel at Mount Sinai, there are three main components: the law, the sacrifice because they couldn't keep the law, and the choice. Essentially, the choice boiled down to this: God said, 'I will bless you if you obey me. I will curse you if you disobey me.' If you obey me and are faithful to this covenant, I will bless you like no nation has ever been blessed. If you are disobedient to this covenant I make with you, I will curse you like no nation has ever been cursed. The world will be astounded at how you can be so cursed and still survive as a nation. They will know God is behind that in some way.

To put it simply, many of those promises for protection and blessing are given in that context. However, we do not relate to God based on the Mosaic covenant. We relate to God not on the old covenant but on the new covenant. While there was a sort of transactional aspect under the old covenant, that is not the arrangement God has made under the new covenant. Under the new covenant, we are not blessed if we obey and cursed if we disobey, other than just in the normal effects of cause and effect in the world. God says, 'No, I will bless you on the basis of faith.' If you are disciplined, it is as a parent disciplines a child. This is the primary reason why you see more promises of material blessing, prosperity, and comfort in the Old Testament; it is in the context of the covenant that God made with Israel at Mount Sinai.