As Paul addresses the Ephesian Church he reminds them that they were dead in their trespasses, but God who is rich in mercy made them alive together with Christ (2:4-5) - it’s by grace that we are saved through faith, not as a result of works so that no one may boast. It is by God's power and work that we are saved, not our own (2:8-9). Then he goes on to inform them that the very reason God did this was to create a people who do His work (2:10). So, in essence, Paul is saying you have been saved BY God's Work FOR God's Work.
Then, in another place, just so that we don't become full of ourselves after conversion, Paul reminds the Philippian Church that even the work being doing is by God who is at work in them both to will and to do according to His good pleasure (Phil. 2:13). So, we are saved BY God's Work FOR God's Work which HE DOES in us and through us. It's all dependent upon Him and all for him.
If we say we do good works to be saved, we've misunderstood the POWER of the gospel. But if we say that we have been saved with no good works as a result, we misunderstand the PURPOSE of the gospel. Then, if we say we've been saved by faith for good works that we now do in our own strength, we've misunderstood the ONGOING nature of the gospel experienced in a walk of faith where we are dependent consistently on the power of God at work in and through us. A full gospel understanding sees God and His work as the beginning, middle and end to all of our salvation.
So, what does this have to do with building a Church? Everything.
Over the years I have observed many leaders in the church, myself included, using Acts 2:42-47 without Acts 1:8 in a similar fashion to using Ephesians 2:10 without Ephesians 2:8-9. We have sought to build the church through good works that produce faith instead of by grace through faith that produces good works.
We do this to varying degrees, but the most frequent example I observe is this: "We want to become an Acts 2 Church." By this we mean, we are going to do what read in Acts 2:42-47 in the hopes of becoming a healthy or successful church. The elements of Acts 2 then become the prescription for a healthy church and if we just get our people to do these things our church will thrive.
What we read in Acts 2 is a beautiful picture of a transformed people, but this is the outcome of God's work, not man's. By God's power, Jesus was building his church, making her a beautiful bride so that what we see is the outcome of Jesus' work. To put this on the Church without Acts 1:8, is like dressing up a dead corpse in Jessica Biel's red carpet dress. The outside fabric resembles something of beauty, but the reeking stench of death is too strong to keep us admiring.
We need to go back one more chapter to Acts 1:8 - You will receive power when the Holy Spirit has come upon you, and you will be my witnesses... The power of God leads to the purposes of God being accomplished, and the purposes of God leads to a distinct kind of people being created by Him and for Him.
I hear many people say their church is committed to an Acts 2 vision of the church, but what is often lacking is the Acts 1 power and purpose for the church. If we start with Acts 1, we believe by faith that Jesus will build his church by the power of His Spirit, not our might. We also believe by faith that it is His Church for His glory, not ours. And lastly, we believe that the church we read about in Acts 2 will be the outcome of a people dependent upon God for power and committed to the purposes of God to be on His mission to be witnesses who are sent to the world.
Certainly we can build Acts 2 churches in our own power and might, but instead of a gospel centered, grace-filled Church, we'll have a me-centered works-based church. And yes, we can work hard to get our people to do the stuff of Acts 2, but will it be for the mission of the gospel to reach to the ends of the earth for God's glory or the mission of our church to become the best and busiest church in town?
A Church that starts and stays in the power and purpose of God in Acts 1:8 will experience themselves becoming the people of God that we see in Acts 2:42-47