Albert Barnes Commentary Haggai 2:4

"Yet now be strong, O Zerubbabel, saith Jehovah; and be strong, O Joshua, son of Jehozadak, the high priest; and be strong, all ye people of the land, saith Jehovah, and work: for I am with you, saith Jehovah of hosts," — Haggai 2:4 (ASV)

Yet now be strong ... and work – They are the words with which David exhorted Solomon his son to be earnest and to persevere in the building of the first temple (1 Chronicles 28:10). Take heed now, for the Lord hath chosen thee to build an house for the sanctuary: be strong and do (1 Chronicles 28:20). Be strong and of good courage, and do. This combination of words occurs only once elsewhere (2 Chronicles 19:11), in Jehoshaphat’s exhortation to the Levites and priests and chiefs of the fathers of Israel (2 Chronicles 19:8), whom he had set as judges in Jerusalem.

Haggai seems then to have adopted the words, with the purpose of suggesting to the downhearted people that there was need of a similar exhortation, in view of the building of the former temple, whose relative glory so depressed them. The word “be strong” (elsewhere rendered, “be of good courage”) occurs commonly in exhortations to persevere and hold fast, amid whatever obstacles.