"As newborn babes, desire the sincere milk of the word, that ye may grow thereby:" 1 Peter 2:2
What causes the transformation in the baby's physical appearance is the milk/formula it takes in. When a baby drinks its "food', it takes in certain nutrients from that food. The nutrients that are in it are what causes the change. As those nutrients get into the muscles, organs, cells and tissues in that baby's body those nutrients start bringing about a change in its body. The more nutrients it takes in the more noticeable the change, until it no longer resembles a baby and eventually changes into an adult.
Our spiritual growth happens the same way- it comes through the "nutrients" we receive from God's word. When we take His word in, it is like taking "spiritual growth hormones". But I am not talking about just casually reading it and then being a "forgetful hearer" of what you read. I am talking about letting His word get down into your spirit and putting it into practice in your life. The putting it into practice and letting it govern your life is what brings about transformation in us. As we read the word and apply the word a transformation takes place in our spirits and there begins to be a noticeable difference in us. If we continue daily to read God's word and live our lives by it the more noticeable that difference will be. So noticeable that we will eventually come into the "measure of the stature of the fullness of Christ." (Ephesians 4:13) In other words, we will come to spiritual maturity and look like Christ.
Have a great day. Our spiritual growth comes from the nutrients we receive from God's word.
For further reading:
James 1:22,25
1 Corinthians 3:1-3
Hebrews 5:14