1 PETER 2:2 As newborn babes, desire the sincere milk of the word, that ye may grow thereby:
Physical and spiritual growth are similar. The basics include activity, food, and time. The Bible supplies our spiritual nutrition.
The essential need for spiritual development is a complete supply of nourishing spiritual food. However, in order to become strong rather than just getting fat, spiritual exercise is also necessary.
JAMES 1:22 But be ye doers of the word, and not hearers only, deceiving your own selves.
Spiritual exercise is doing what the Bible says. For instance, Sharing God's Word, praying, appreciating God, expressing gratitude, acting in love, and believing in God.
EPHESIANS 4:15 But speaking the truth in love, may grow up into him in all things, which is the head, even Christ:
The spoken Word of God is important for spiritual development. God's Word about you needs to be customized and internalized. Your thoughts, perceptions, and actions will then start to align with God's Word. It's called Bible contemplation.
1 TIMOTHY 4:15 Meditate upon these things; give thyself wholly to them; that thy profiting may appear to all.
How will others and youself realize when you are having spiritual growth?
Spiritual fruit is a sign of spiritual development. So, to evaluate whether you are developing spiritually, look into your life for more of the fruit mentioned in Galatians 5:22.
GALATIANS 5:22-23 But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, 23 Meekness, temperance: against such there is no law.
Keep in mind that development takes time hence don't be losing courage if you fail to see bountiful spiritual fruit right away. Continue to digest God's Word, it will enrich the fruit of growth.
Being more like Jesus in your behavior is another way to define spiritual fruit (and subsequently, spiritual development).
EPHESIANS 4:13 Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ:
You will develop your faith and love while your spirituality grows.
2 THESSALONIANS 1:3 We are bound to thank God always for you, brethren, as it is meet, because that your faith groweth exceedingly, and the charity of every one of you all toward each other aboundeth;
SAY THIS: I adore the pure milk of God's Word because it nourishes my spirit and makes me more like Christ.

