Growth is not an instant process, it is a life-long event. Just imagine our physical growth as humans, it is not just in a snap that we transformed from infants to adults. Same thing with spiritual growth, it is not just a snap that we will be matured right away. It is also a life-long process. God is teaching us things and revealing to us some circumstances day by day.
Once we accept Christ as our personal Lord and Savior, we are a new creation. Our past was gone and the new has come (2 Cor. 5:17). It is also from milk to meat. How to be spiritually matured? Just read the Bible, pray everyday, and you will grow, as an old children christian song said. That old song is a very simple reminder on how to become a matured christian.
Keep your hearts in tune with the Lord. Our spiritual journey is indeed a process. It is a labor of a lifetime. What a great privilege that God is always in our lives. Let us grow in the knowledge of our Lord and Savior Jesus Christ (2 Pet. 3:18).