Albert Barnes Commentary Haggai 1:12

"Then Zerubbabel the son of Shealtiel, and Joshua the son of Jehozadak, the high priest, with all the remnant of the people, obeyed the voice of Jehovah their God, and the words of Haggai the prophet, as Jehovah their God had sent him; and the people did fear before Jehovah." — Haggai 1:12 (ASV)

Then Zerubbabel, and all the remnant of the people—not "the rest of people" but "the remnant," those who remained from the captivity, the fragment of the two tribes, which returned to their own land—hearkened unto the voice of the Lord. This was the beginning of a conversion. In this one thing they began to do what, all along in their history, and most in their decay before the captivity, they refused to do: obey God’s word.

So God sums up their history by Jeremiah (Jeremiah 22:21): I spake unto thee in thy prosperity, thou saidst, I will not hear.

This is thy way from thy youth, that thou hearkenedst not unto My voice.Zephaniah 3:2 states, still more briefly, she hearkened not unto (any) voice. Now in reference, it seems, to that account of their disobedience, Haggai says, using the very same formula, they hearkened unto the voice of the Lord, according to the words of Haggai. They obeyed, not vaguely, or partly, but exactly, according to the words which the messenger of God spoke.

And they feared the Lord - “Certainly the presence of the Divine Majesty is to be feared with great reverence.”

“The fear of punishment at times transports the mind to what is better, and the infliction of sorrows harmonizes the mind to the fear of God; and that of the Proverbs comes true (Proverbs 13:13): He that feareth the Lord shall be recompensed, and (Proverbs 19:23) the fear of the Lord tendeth to life; and Wisdom : The fear of the Lord is honor and glory, and (Proverbs 19:12) the fear of the Lord shall rejoice the heart, and giveth joy and gladness and a long life. See how gently and fittingly God smites us.”

“See how the lovingkindness of God immediately goes along with all changes for the better. For Almighty God changes along with those who choose to repent, and promises that He will be with them; what can equal that? For when God is with us, all harm will depart from us, and all good will come to us.”