It is customary for the theologians to define the Church as a gathering where the Word is preached and the Sacraments performed. These are the two marks of the Church.
My own take on the discussion is that this isn’t enough. If Kingdom morphs into Ecclesia, then we need to add a third “mark”: a missional community. We don’t just gather to hear the Word and observe the sacraments, but we gather as a missonal community to witness to that mission and to strategize and prepare for further mission and to be formed through fellowship.