Haggai’s first message to the people was that God was frustrating their lives. Why? because they were delaying the building of the temple. The Returnees had come back nearly 20 years before, had stated and stopped because of opposition. In the mean time they had taken care of their own houses. So the prophet said that God was making it so they were frustrated at every turn. In the words of Haggai 1:6, they earned wages only to put them in a purse with a hole in it.
The OT covenantal background is found in such passages as Leviticus 26:19-20 and Dt 28:22-24. The people would be materially blessed or cursed based on their faithfulness.
So that is clear enough.
However, how do we read this from the New Testament point of view? Should preachers use this to motivate building projects and giving campaigns? The Health and Wealth preachers speaking of giving to God to get greater wealth back. ( As of the Lord is a sort of E-Trade agent.)
The difference is that the Temple in the NT is sometimes the People of God – I Peter 2:4ff. And at other times we see that Christ is the fulfillment of the Temple and all that was related to it (book of Hebrews).
So a Christian interpretation of Haggai 1 has to go through those changes. The temple was a place to meet with God, where the High Priest prayed for the people and offered sacrifices for sin and for fellowship. It was where the glory fo God resided. All of those are fulfilled in Christ who is the means by which we come to God, he is the High Priest, he is the Sacrifice, he is the Glory of God revealed.
So how does Haggai 1 preach today? It seems to me that it is summarized very well by Jesus in the Sermon on the Mount – “Seek first the kingdom of God, and all these [material] things will be given to you as well.”