Well, you’ve asked a profound question! In a sense, the whole New Testament is the answer. But if we are looking for character qualities, it would be hard to do better than the fruit of the Spirit in Galatians 5:22-23. Maturity to me means having a “grown-up” faith that has been tested and made strong in the trials of life.

Years ago I heard a Bible teacher make a helpful distinction between being “spiritual” and “mature.” A new-born Christian is “spiritual” but not yet “mature.” That is, the new Christian knows the Lord but has not yet developed godly character that comes only over a long period of time. A mature Christian to me would be a person whose character has been made strong over time so that they display the nine-fold fruit of the Holy Spirit.

No one does this perfectly because maturity does not mean perfection. But if being a Christian means to become like Christ, then I think the fruit of the Spirit is a good place to start.