The LORD uses the command “Be Strong” three times. He first tells Zerubbabel, the highest ranking governmental leader; then, Joshua, the highest ranking religious leader; and lastly all the people of the land. All the people were discouraged and afraid of failing. God left no excuse for anyone to not be strong when he said it to even the highest officials. Of course, then, it would apply to the rest of the people.
Anytime a prophet/biblical author uses the command be strong, one is reminded immediately of the call to the Hebrews (Deuteronomy 11:8), then to Joshua and the call of the Lord on him, which he passed on to the people God called him to lead:
Deuteronomy 31:23 ESV And the LORD commissioned Joshua the son of Nun and said, "Be strong and courageous, for you shall bring the people of Israel into the land that I swore to give them. I will be with you."
Joshua 1:6-7 ESV Be strong and courageous, for you shall cause this people to inherit the land that I swore to their fathers to give them. Only be strong and very courageous, being careful to do according to all the law that Moses my servant commanded you. Do not turn from it to the right hand or to the left, that you may have good success wherever you go.
Albert Barnes wrote,
"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 Ch. 28:10. “Take heed now, for the Lord hath chosen thee to build an house for the sanctuary: be strong and do” 1 Ch. 28:20. “Be strong and of good courage, and do.” This combination of words occurs once only elsewhere 2 Ch. 19:11, in Jehoshaphat’s exhortation to “the 2 Ch. 19:8 Levites and priests and chiefs of the fathers of Israel,” whom he had set as judges in Jerusalem. Haggai seems then to have adopted the words, with the purpose of suggesting to the down-hearted people, that there was need of the like 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…"Be strong. Be of good courage as the phrase is also spoken other places in the Old Testament. When you have the fear of failing and being inadequate for any situation whether it concern family, work, or church, BE STRONG!
Today we are also in a rebuilding process. Many of our lives are being rebuilt. Your life may have been destroyed by divorce. Your job may have been destroyed by lay-off. Your church may have been destroyed by conflict. Whatever the LORD is rebuilding in you, remember that you must be strong and of good courage.
Understand that this is not your own strength. Your strength is not a mere physical or mental strength. This is not the sort of strength that a world chess champion uses when strategizing against the opponent. This is not the sort of strength that an offensive linemen uses to keep the defender out of the backfield. This is not the sort of strength that Mark McGuire used to belt balls out of stadiums …not even Mark McGuire on steroids describes this kind of power.
Zechariah 4:6 ESV Then he said to me, "This is the word of the LORD to Zerubbabel: Not by might, nor by power, but by my Spirit, says the LORD of hosts."