The concept of the church as the bride is from John's vision of the "wedding feast of the Lamb" in Revelation 19 . It is when the church is finally united with Christ in Heaven:
7 Let us be glad and rejoice,
and let us give honor to him.
For the time has come for the wedding feast of the Lamb,
and his bride has prepared herself.
8 She has been given the finest of pure white linen to wear.”
For the fine linen represents the good deeds of God’s holy people.
9 And the angel said to me, “Write this: Blessed are those who are invited to the wedding feast of the Lamb.” And he added, “These are true words that come from God.”
Revelation 19:7-9 NLT