There maybe several interpretations of the key of David (3:7). For example, David listened to and was devoted to the Lord, and he had a zest for life. This maybe a key of David. In regard to the Church, however, perhaps the main Key of David in Revelation 3:7 was David's desire to build a temple (2 Samuel chapter 7). God didn't want a temple, He said: "...Heaven is my throne and the Earth is my footstool..." (Isaiah 66:1). "...the Lord himself will establish a house for you...your offspring...will build a house for my name..." (2 Samuel 7:11-13).
David assumed the Lord meant that his son Solomon would build the temple. And David gathered material for a temple. Since David was determined, God relented and gave Solomon plans for a temple--a very magnificent temple! Details of the temple are in: 2 Samuel 7; 1 Kings 5-8; 1 Chronicles 17, 22, 28, 29 and 2 Chronicles 2-7, 24.
"Yet a time is coming and has now come when the true worshipers will worship the Father in spirit and truth, for they are the kind of worshipers the Father seeks." (John 4:23, also note verses 21-24). Throughout history, the temple and churches (although very helpful) have hindered us from viewing all of God's creation (the body of all that God created) as the Temple of God.* We were created in the image of God (Genesis 1:27). We maybe a model of all that God created.
Jesus talked of himself as a temple John 2:19-21 (also see: Matthew 26:61 and Mark 14:58). So writers of the New Testament believe the builder of the "house," that God told David about, was Jesus Christ. Each believer is a part of The Body of Christ (1 Corinthians 12:27).
Words can be confusing at times. The whole universe is God's Temple. See Acts 17:24+. Jesus Christ is the temple (body or house) at the heart of God's Temple (all of creation). Each person has an unique place in The Body of Christ. Perhaps the embryo theory will aid in understanding.
*This is so important, that it will be repeated several times. It seems that temples and churches tend to place blinders on us -- so we don't see the temple God has created for us, which is all of God's creation! Each of us worships God by what we do in our daily activities, including how we acquire and use: every penny of our money; every second of our time; and, every other resource at our disposal.