This is the word of the Lord unto Zerubbabel. The Lord's message unto Zerubbabel is the purport of the vision, viz. that his work will be accomplished through the grace of God alone. Not by might. Septuagint, "not by great might;" but the Vulgate, "not by an army."
The word is almost synonymous with the following, translated power; and the two together mean that the effect is to be produced, not by any human means, however potent. Doubtless Zerubbabel was dispirited when he thought how much there was to do, how feeble the means at his disposal (Nehemiah 4:2), and how formidable the opposition; and nothing could better reassure him than the promise of Divine aid.
But by my Spirit. The angel does not say expressly what is to be done; but the purpose that filled the minds of Zechariah and Zerubbabel applied the word. The operations of the Spirit are manifold, and his aid alone could bring these mighty things to pass.
The oil is a figure of the grace of the Holy Spirit; and as the lamps are not supplied by human hands, but directly from the olives, so the good work now undertaken shall be supported by Divine means (see on verse 14).