"And those who are far off shall come and help to build the temple of the LORD. And you shall know that the LORD of hosts has sent me to you. And this shall come to pass, if you will diligently obey the voice of the LORD your God."
This word of prophecy from Zechariah to the returning exiles in Jerusalem contained a certainty and a contingency. The certainty was that further help was coming in the temple project. You can read all about it in detail in the contemporary account found in the book of Ezra. God was re-gathering the remnant of His people. Jerusalem was once again becoming repopulated with returning Jews from Babylon. And the temple would be rebuilt. God would be glorified and His Word confirmed by these events.
But this was all contingent on obedience. And the returned exiles did struggle to do this. You can read all about that in another contemporary of Zechariah, the prophet Haggai, who had to compel the Jews to finish the work they had started. God would bring the blessings of His worship to His people if they would obey Him in the re-building of the temple. Diligent obedience and hard work would have to come on their part for the blessing to be truly known to the nation.
Obedience to the Lord is often required for us to know His full blessing. To the believer in Jesus, sanctification involves living out what God has given us in Christ. We are declared holy, but we must learn to make holy choices if we are to truly know God and please Him. God’s blessing is still offered, but the “if” of our own holy choices and obedience often must precede the “then” of God’s blessing and promise to us.