Invisible Church
One key idea in the theology of John Calvin was the "Invisible Church" vs. the "Visible Church." According to Calvin, the "Visible Church" included the institutions of the church, official church membership, etc. These were signs of the earthly church that could be measured and quantified.
The "invisible church," however, included all those who had been chosen for salvation. This group, Calvin asserted, was unknowable. We humans cannot know whom God has selected for salvation. So, trying to discern the membership of the invisible church made no sense for Jesus' followers. That is a decision by God, left up to God.
The parable of the Wheat and Weeds affirms Calvin's teaching. We humans aren't ever in a position know who exactly is wheat and who is weed. Jesus will sort that out. We are called to trust Jesus to figure it all out and to be faithful to him.