The sign of Christian maturity is not that you are 60 years old, or that you’ve been a member of a church for 40 years; nor is it that you’re a leader in the church. The sign of Christian maturity is that there is no doubt whatsoever in your mind, or in anyone else’s mind, that preeminent in your life is the Lord Jesus Christ.
So are you mature?
You could be 20 years old and mature on this sense. You could be 70 years old and spiritually an infant in this sense.
When you understand that in Jesus Christ there are hidden and available to us all the treasures of wisdom and knowledge you will bend every back, you will strain every nerve, you will engage in all kinds of struggle — and it is a struggle isn’t it? — to do all you can that Christ will be preeminent, that his people will be encouraged, that we will be united in love, instructed in Christ, and not deluded by any second rate offer of salvation.
— Sinclair Ferguson in a sermon on Colossians 1:24-2:5