Jamieson, Fausset and Brown on Zechariah 8:6

6. However impossible these things just promised by Me seem to you, they are not so with God. The "remnant" that had returned from the captivity, beholding the city desolate and the walls and houses in ruins, could hardly believe what God promised. The expression "remnant" glances at their ingratitude in rating so low God's power, though they had experienced it so "marvellously" displayed in their restoration. A great source of unbelief is, men "limit" God's power by their own (Psa 78:19, 20, 41).

these days--"of small things" (Zec 4:10), when such great things promised seemed incredible. Maurer, after Jerome, translates, "in those days"; that is, if the thing which I promised to do in those days, seems "marvellous," &c.