One of the amazing things about children is their ability to teach you things about the divine. I am a closet Barthian on epistemology so I know these types of analogical posts must have him rolling in his grave. But there is an amazing value in being a parent to little children.
My children take great joy in the smallest things. And I enjoy their joy. Christian obedience is something like this. God, through His Spirit, works in us the smallest of obedience. This becomes the smallest of pleasures. And it pleases Him.
I cannot stress this strongly enough. Christian obedience develops much like children. It is the small things that delight our souls and please our Father. These things grow and grow. Pastors and Elders can excite us with the possibilities of walking, running, and riding a bike. The Lord is pleased with us sticking our tongue out and laughing.
This is what it means to grow in the Lord. It cannot mean that we start as mature adults. It refers to natural growth. There is an element in which the spiritual follows the physical in this time. We begin craving milk. We move towards meat.
Many arguments over sanctification would be resolved by participants acknowledging that they are still young babes. Much chastising would be avoided if mature Christians would tend to the young believers in Christ. The joy of our Father consists in watching His children grow in the full mature figure of Christ. This is solely because of Christ's atonement. But it does not diminish the fact — God is pleased to be our Father in every regard. This is the source of our joy.