Rashi quotes the Midrash which says that when the nations of the world saw the tabernacle, they regarded it as testimony that God had forgiven His people for their lapse with the Golden Calf. Commenting on Rashi, the Sif’tei Chachamim says that Israel had rehabilitated themselves after the sin and gone ahead to create a sanctuary that showed their loyalty to God, because even if a Jew sins he is still a Jew, and in the internal tug-of-war the Jew in him overcomes the sinner.
Since the question remains of which point it was when it was clear that God had forgiven the people, one possible answer is that building the sanctuary in itself was already a sign of Divine forgiveness. However, simply putting up a building is not enough, and the mark of forgiveness was when God’s Presence entered the edifice and brought it to life.
We can use this as an analogy to building a synagogue. A community know that the synagogue has become a House of God not when the builders lay down their tools but when the Divine Presence in the sanctuary becomes palpable. The synagogue needs more than bricks and mortar. It needs to pulsate with spirituality and dedication.