Spiritual maturity is measured by one’s likeness to Christ. Of course, it goes (almost) without saying that no likeness to Christ is possible at all as long as a person has no real faith-based relationship with him. That is, spiritual maturity is not even an issue when one is spiritually dead.
For a Christian, though, likeness to Christ is the goal of our election (Romans 8:29) and the purpose of the work of the Holy Spirit in us. Viewed from another angle, spiritual maturity is measured the same way the maturity of a fruit tree is, by the amount and quality of its fruit, specifically what the Bible calls the fruit of the Spirit; and we’re not talking about some hyper-spiritualistic drama; rather, the Bible tells us what the fruit is that the Holy Spirit works in us; Love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, and self-control. Those are the measure of spiritual maturity.