For those reading closely, this raises another question: What if the sheep comes back? Jesus seems to answer that question with another parable, this one concerning an unforgiving servant (18:21–35). The point here is simple: We forgive those who have sinned against us. Why? Because we’ve been forgiven by the God whom we’ve sinned against, an offense far more severe than whatever slights we’ve endured.
In other words, pastors—no, in other words, churches—we quickly and gladly and wholly forgive returning and repentant sheep because we know we ourselves have strayed and, if not for God’s tether on us, we’d stray again and again, farther and farther. Mirroring David in Psalm 23, this hymn describes the lot of us:
Perverse and foolish oft I strayed,
But yet in love He sought me,
And on His shoulder gently laid,
And home rejoicing brought me.
—“The King of Love My Shepherd Is” (Henry Williams Baker, 1868)
In summary, Matthew 18 teaches us both the foundation and trajectory of church discipline: We pursue straying church members because God pursues his lost sheep, even if it’s “just” 1 of 99. Sadly, this will occasionally result in exclusion because some lost sheep intend to stay lost. We will give them what they ask for and let them go, but we will insist on speaking honestly as they do.
Happily, however, lost sheep have a way of coming back—and when they do, we should forgive them swiftly and completely because God in Christ has forgiven us swiftly and completely.
Text #2: Hebrews 10:23–25 
Here are the verses in question:
Let us hold fast the confession of our hope without wavering, for he who promised is faithful. And let us consider how to stir up one another to love and good works, not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near.
The author of Hebrews has two commands for us. The first is in verse 23: Hold fast to the confession of our hope, a confession he’s just spelled out for us by extolling what Christ accomplished for us as our high priest. This command is rooted in the faithfulness of God (verse 23).
Thankfully, the second command—stir up one another to love and good words—is accompanied by an immediate application. How do we do this? Simple: We keep on meeting together. Why? Because we can’t encourage someone we never see. Again, the author roots this command and its application in a promise: We gather and encourage and spur on because we see Judgment Day drawing near, when our faithful, promise-keeping God will return and we will gather with him, forever.
Though he wrote nearly two millennia ago, the author of Hebrews seems familiar with our modern predicament. Did you notice? “Not neglecting to meet together, as is the habit of some.”
It is indeed the habit of some Christians to neglect meeting together. In doing so, they miss out on encouragement; they miss out on being spurred on to love and good works. But that’s not all: Their vantage point on God’s work in the Christian life shrinks, their confidence in their confession of hope wanes, their memory of God keeping his promises fades, and their once clear-eyed vision of the coming Day of the Lord blurs to black.
Speaking of, did you notice how severe this warning is? The Day of Judgment? Explain to me, then, how removing someone from membership is too severe. Imagine a non-attending “church member” arrives at Judgment Day and is told eternal judgment awaits. At this moment, how “loving” will that church seem who did nothing, or who quietly deleted his name from a computer? Will he not be right to be angry at that church: “Why didn’t you warn me?”
In fact, our small, two-dimensional pictures of removal now may be the most loving thing we can do because they warn people of the potential permanent reality of removal to come.
These verses in Hebrews let us pursue non-attending members with our Bibles open to a chapter and verse, rather than a list of well-intentioned, thought-through suggestions. We can point not only to a violation of a biblical command, but also to the God-ordained benefits they’re missing.
Text #3: Hebrews 13:17 (Acts 20:28)