Albert Barnes Commentary Zechariah 8:9

"Thus saith Jehovah of hosts: Let your hands be strong, ye that hear in these days these words from the mouth of the prophets that were in the day that the foundation of the house of Jehovah of hosts was laid, even the temple, that it might be built." — Zechariah 8:9 (ASV)

Let your hands be strong - The fulfillment of God’s former promises is the guarantee of future ones; His past providences are a token of those yet to come. Therefore, having such great promises for the future, they were to be diligent in whatever God gave them to do in the meantime.

He speaks to them, as hearing in these days, that is, the fourth year of Darius in which they apparently were, these words from the mouth of the prophets, which were in the day when the foundation of the house of the Lord was laid, the temple, that it might be built.

Haggai had now gone to his rest; his voice had been silent for two years, but his words lived on. The fulfillment of what the prophets had then spoken in God’s name was a reason why their hands should be strong, both then and afterward, for every work God gave or would give them to do.

Ribera: “Some things are said to Jerusalem, that is, to the Jews, which belong to them only; some relate to what is common to them and the other members of the Church, that is, those who are called from the Gentiles. Now he speaks to the Jews, but not so as to seem to forget what he had said before. He would say, You who hear the words which Haggai and Zechariah spoke in those days when the temple was founded, be strong and proceed to the work you began of fulfilling the Lord’s will in building the temple, and in avoiding the sins in which you were previously entangled. For just as, before you began to build the temple, you were afflicted with many calamities, but after you had begun, all things went well with you, as Haggai said, ‘so, if you cultivate piety and do not depart from God, you shall enjoy great abundance of spiritual good’ (Haggai 2:15–19).”

Osorius: “The memory of past calamity made the tranquility of that time much sweeter and stirred the mind to greater thanksgiving. He then set forth the grief of those times when he says:”