The apostle Paul spoke for all pastors when he reminded the Corinthians of the difficulty and burden of his ministry — imprisonments, beatings, stonings, shipwrecks, long journeys, robbers, and hard work. Yet the greatest source of his concern was “the daily pressure on me of concern for all the churches. Who is weak without my being weak? Who is led into sin without my intense concern?” (2 Cor. 11:28-29)
One of a faithful pastor’s greatest burdens is his spiritual concern for those who struggle with the faith and or ultimately leave the faith. There is no greater sorrow than to see individuals walk away from Christ.
Yet, as John notes in 2 John 2:19, God has a gracious reason for these people leaving the church. When John says, “They went out so that it would be shown that they are not all of us,” he means that someone had a purpose in their leaving the church (and that person is God). When unbelievers leave Christ and the church it makes it clear who is a believer and who is not. When they reject Christ by lifestyle and words, it is evident what they are. And that is a grace because then they go from being “one of us” to being a mission field. And then we can begin communicating the gospel to them, something we might not be bold to do if they stay in the church as unbelievers.
But there is also another grace for having unbelievers leave the church — it becomes a point of assurance for those who remain. Notice what John says in this verse: “if they had been of us, they would have remained with us.” I can’t say that every single person that joins a church and is a regular attender of worship and active in doing ministry and service is a believer (because I cannot see the heart). But in general, when you see someone who stays in the church and fights for his faith and continues in ministry even when it is hard and maintains relationships, that person is a believer. Remaining in the body is an encouraging means of assurance. And the corollary to that statement is that when a person isn’t interested in fighting against sin and isn’t interested in worship and God’s people and either leaves church entirely or starts his own sect or cult, he isn’t a believer.
The formal theological word for this is perseverance. A mark of believers is that they persevere with Christ. They don’t give up on the faith. It may be a struggle at times; the spiritual disciplines and battle against sin may be hard, but they don’t quit. They keep moving toward Christ because they know that only in Christ is life.
The New Testament repeatedly affirms that God’s people — genuine Christians — will always stay true to the faith and persevere to the end (e.g., Jn. 10:27-30; Phil. 1:6; 1 Pt. 1:5; Jude 24-25). Yes, true Christians will still sin, and may temporarily engage in a pattern of sin. True Christians will always struggle against sin and may struggle against one or more sins all their lives, but they do not give into sin permanently and they do not become antichrists.
This is commonly called “eternal security,” but that can imply that one can sin without consequence — that salvation is a means of having liberty to sin. Yes, the genuine believer is completely secure in his salvation; but a better way to say that is that a genuine believer will persevere (continue) in that salvation. He will be imperfect and his progress will be incomplete, but he will continue moving toward Christ and will continue living his salvation.
As John Stott has said, “Future and final perseverance is the ultimate test of a past participation with Christ” (Heb. 3:14).