What is spiritual growth and why are so many Christians concerned atheists lack it?

The sign that we are growing in Christ is not found in our ability to suppress sins or do more good by human effort. Even unbelievers do good things and can say no to sin given the right circumstances. The sign of our maturity is found in the revelation of how much we are loved and forgiven by God in Christ, how we are/can do nothing without Christ and how bogged down and entrenched in sins we would be were it not for the grace of our Lord Jesus Christ.

See Paul's own revelation as he grew in grace and according to the sequence of letters that he wrote to the church, the first step being from his description of himself as "the least of the apostles" in 1 Corinthians 15-

"For I am the least of the apostles, and not fit to be called an apostle, because I persecuted the church of God. 10 But by the grace of God I am what I am, and His grace toward me did not prove vain; but I labored even more than all of them, yet not I, but the grace of God with me."

In a later epistle, Ephesians 3, he described himself as "less than the least of all the Lord’s people," down from the least of the apostles -

"7 I became a servant of this gospel by the gift of God’s grace given me through the working of his power. 8 Although I am less than the least of all the Lord’s people, this grace was given me: to preach to the Gentiles the boundless riches of Christ,"

Again in a later epistle, 1 Timothy 1, he went down further seeing himself at the bottom of the rung of of all sinners- the worst of them all!

"15 This is a trustworthy saying, and everyone should accept it: “Christ Jesus came into the world to save sinners”—and I am the worst of them all. 16 But God had mercy on me so that Christ Jesus could use me as a prime example of his great patience with even the worst sinners."

As Paul increased in the knowledge of God and of our Saviour Jesus Christ, he saw more and more who he would be without God's saving Grace- from the "least of the apostles" to the "less than the least of all the Lord’s people" and finally the worst of all sinners. He attributed all that he achieved to the Grace of God within him; the same Grace that we have today. Beloved of God, God's Grace is a Person. His name is Jesus Christ. Titus 2:11 says of Him:

"God’s marvelous grace has manifested in person, bringing salvation for everyone."

Verse 14 of the same Titus 2 says: "He gave His life to free us from every kind of sin, to cleanse us, and to make us His very own people, totally committed to doing good deeds."

Believers, never ever think that you can attain sinless perfection or even have victory over any sin by you own will-power aka arm of flesh that will fail. Never attrribute your success and victories to your own smarts. The reason why we are not wallowing in heinous sins right now is because God did not allow us to be tempted beyond what we can bear and with every temptation Christ is the Way of escape for us. With this knowledge through Christ, it becomes difficult for us to condemn those who are yet to receive their invitation to enjoy God's abundance of grace and His gift of righteousness "…for all who receive it will live in triumph over sin and death through this one Man, Jesus Christ." Romams 5:17.

The flesh (human nature) in a pastor is the same flesh that was in Hitler. Given the vagaries of Time and Chance as well as Satan's sifting as he did Peter who absolutely knew that he could not deny Christ, of ourselves, anyone can commit the most heinous of sins. God's mercy is the skeleton that is restraining man from crumpling into the helpless mass of sins-ridden flesh that man is of himself.

Sin in the flesh (human nature) is activated when we try to keep the law (see 1 Cor 15:56) or think that some sins are beneath us because we are "better than that." This is why Peter could deny Jesus when he absolutely knew that he would not. He swore that he wouldn't but ended up doing the very thing that he swore not to do...three times...and with swearing and cursing.

Believer, is there any sin or addiction that you are still trying to suppress with your own power or think you can handle like Peter? The arm of flesh (human effort at keeping the law/trying to be godly) will fail and when it does, those suppressed sins will burst forth in a volcanic eruption of sinful deed that lead to death. See Romans 7-

“4 So, my dear brothers and sisters, this is the point: You died to the power of the law when you died with Christ. And now you are united with the one who was raised from the dead. As a result, we can produce a harvest of good deeds for God. 5 When we were controlled by our old nature, sinful desires were at work within us, and the law aroused these evil desires that produced a harvest of sinful deeds, resulting in death.”

The one with the old nature is not a new creation in Christ Jesus. He does not believe Romans 5:9-

“And there is still much more to say of His unfailing love for us! For through the blood of Jesus we have heard the powerful declaration, “You are now righteous in My sight.” And because of the sacrifice of Jesus, you will never experience the wrath of God."

He still tries to get right with God by keeping the law but in God's Wisdom which is illogical to man's reasoning, this very act of trying to attain godliness (be like God) by human effort causes sin to proliferate. Adam and Eve followed this seemingly good but deathly and antichrist doctrine of Satan and look where it got them. Beloved of God, stop trying and start believing Jesus by His Grace. This is why He came for you: Matthew 1:21-

And she will have a Son, and you are to name Him Jesus, for He will save His people from their sins.”

In Christ, our path can only shine brighter and brighter…, not by our own light (righteousness from works of the law) but by our Saviour Jesus Christ, the Light of the world. When we let go (of our puny efforts at saving ourselves and being righteous) and let Him, we can't but grow in grace. Growing in the Spirit is trusting less in ourselves and more in Christ. He is all that we need to live a life of moral excellence.

None of self: all of Christ. This is the Good News.