Then spake Haggai the LORD'S messenger in the LORD'S message unto the people, saying, I am with you, saith the LORD. Then spake Haggai - He was the Lord's messenger, and he came with the Lord's message, and consequently he came with authority. He is called מלאך יהוה malach Yehovah, the angel of Jehovah, just as the pastors of the seven Asiatic churches are called Angels of the Churches, Revelation 1:2.
I am with you, saith the Lord - Here was high encouragement. What may not a man do when God is his helper?