John Piper, in a sermon from Zechariah titled, There Shall Be a Fountain Opened:
We began by suggesting that the main point of Zechariah’s prophecy is, “Fear not, for God purposes to do you good.” A very profitable way to read this book is to mark every verse where God says he is going to do something good for Jerusalem. I marked well over 50 verses. But since we can’t look at all these, let’s focus in on the most important of all: 13:1. I call it most important because all the other benefits promised to Israel (and to us) depend on this one.
“On that day there shall be a fountain opened for the house of David and the inhabitants of Jerusalem to cleanse them from sin and uncleanness.”
Zechariah promises the people that at some future time a fountain would be opened which would take away their sin and guilt. I say this is the foundation for all the other blessings promised because the only way sinners can hope to inherit the riches of God is if their sins are forgiven. The fountain of cleansing is the first checkpoint on the road to heaven.
To understand this Advent promise in the context of Zechariah, I want to try to answer three questions about it: first, why did a fountain still have to be opened? Second, how does this fountain bring about forgiveness? Third, for whom does this fountain provide cleansing?