Christian maturity is being a responsible son or daughter of God. The mature in Christ are people who have stopped being concerned about their own needs and pursuits and have entered into the global vision of their Father so that they may transform a hurting world. The mature go out as His agents in order to accomplish the aims of the Lord's Prayer, "Your kingdom come, Your will be done on earth as it is in heaven" (Matthew 6:10).
It's like a son who is being brought into the family business. Instead of racing fast cars and running around with girls, he finally buckles down and says, "Dad, I'm part of it. It's my business, too, and I'm going to work hard and undertake the burden of this work." That is real maturity.
We undertake the burden of God in the way Jesus Christ would do it. Not in the energy and heat of our own flesh, striving to do something ourselves, but in the anointing of the Spirit of God, by the power of the Holy Spirit.