It is a sin to be stagnant in spiritual growth. The apostle Paul said,
“Not that I have already obtained this or am already perfect, but I press on to make it my own, because Christ Jesus has made me his own. Brothers, I do not consider that I have made it my own. But one thing I do: forgetting what lies behind and straining forward to what lies ahead, I press on toward the goal for the prize of the upward call of God in Christ Jesus” (Philippians 3:12-14).
We always need to remember that we are saved and sanctified by faith alone in Christ alone, but true faith lives and grows. The Christian life is not a self-improvement project, but those who embrace the gospel cannot remain the same, they “press on.”
We are not done until we are complete in Christ. We long for the pure milk (of the Word) so that “by it [we] may grow up into salvation” (1 Peter 2:2). We “make every effort to supplement [our] faith with virtue, and virtue with knowledge, and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, and godliness with brotherly affection, and brotherly affection with love. For if these qualities are [ours] and are increasing, they keep [us] from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ” (2 Peter 1:5-8).
To be sure, growth is often slow, but slow moving is not stagnant. Growth may not be immediately obvious, as with the physical maturation process, overnight is rarely long enough to observe measurable progress. But regular check-ups, perhaps with the help of others who know what to look for, may be quite important, especially if faith and hope and affections are not stronger. Every believing pilgrim makes progress and sins by staying in the same place.