"Then we will no longer be infants, tossed back and forth by the waves, and blown here and there by every wind of teaching and by the cunning and craftiness of men in their deceitful scheming." NIV translation
Now, we see another reason for growth and that is so that we will be able to tell truth from lies. It seems that today there are so many people trying to twist the gospel to fit their purposes which usually include making a bunch of money. We need to grow in our relationship with Jesus Christ so that we will be able to stand firm in the truth and not be swayed by these slick tongued imposters of light.
"Instead, speaking the truth in love, we will in all things grow up into him who is the Head, that is, Christ." NIV translation
God is truth and it is contained in His word. Today, we have a mixed up way of thinking in that we do not proclaim the truth in love but instead we ignore the truth so as not to hurt someone's feelings.
We see here that this stunts the growth of His children and they remain as little children instead of maturing into true disciples of Jesus Christ. He is in charge of all things and His way is truth.
"From him the whole body, joined and held together by every supporting ligament, grows and builds itself up in love, as each part does its work." NIV translation
Jesus brings us together into one body that is His and He is the glue that holds us all together. Just like a physical body, if a part is not functioning properly, you are sick and the whole body is affected. If we do not do our part here on earth, the body of Christ suffers. If we do not tell others about Jesus the Savior, how will they hear and come to be a part of the body of Christ? If we do not train them to be disciples, how will they be an effective part of the body? We do all of this out of love for Jesus and our fellow man.
"So I tell you this, and insist on it in the Lord, that you must no longer live as the Gentiles do, in the futility of their thinking." NIV translation
Paul is telling us that it is not enough to learn to be the light; we need to be the light. That means that we need to be different than those who don't know Jesus. They live according to their own thinking of what is best while we are to live according to God's word which is truly best. On our own, we will not be able to do this but God has given us the Holy Spirit who sanctifies us.