How does the Bible define "spiritual maturity"? Here are some pertinent Bible passages that discuss this important topic.
“And he [Jesus] gave the apostles, the prophets, the evangelists, the shepherds and teachers, to equip the saints for the work of ministry, for building up the body of Christ, until we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ, so that we may no longer be children, tossed to and fro by the waves and carried about by every wind of doctrine, by human cunning, by craftiness in deceitful schemes. (Ephesians 4:11–14)
“Remember your leaders, those who spoke to you the word of God. Consider the outcome of their way of life, and imitate their faith. Jesus Christ is the same yesterday and today and forever. Do not be led away by diverse and strange teachings, for it is good for the heart to be strengthened by grace, not by foods, which have not benefited those devoted to them.” (Hebrews 13:7–9)
Notice that spiritual maturity, according to scripture, is marked by the ability to NOT be carried or lead away by strange winds of false doctrine. But, being so thoroughly grounded in God's Word that you are not deceived.
Put another way, one of the clearest signs that a person is spiritually immature (or even spiritually dead) is the fact that they succumb to and are lead away by every strange wind of false doctrine.
Another thought to consider from scripture regarding spiritual maturity
Pastors, according to scripture are to be models of spiritual maturity in both life and doctrine. Here are the qualifications of a pastor as laid out in the Pastoral Epistle, Titus. Pay close attention to what the Holy Spirit reveals regarding the vital importance of sound doctrine and how those who will not abide by sound doctrine and teach falsely are "detestable, disobedient, unfit for any good work".
“Paul, a servant of God and an apostle of Jesus Christ, for the sake of the faith of God’s elect and their knowledge of the truth, which accords with godliness, in hope of eternal life, which God, who never lies, promised before the ages began and at the proper time manifested in his word through the preaching with which I have been entrusted by the command of God our Savior;
This is why I left you in Crete, so that you might put what remained into order, and appoint elders in every town as I directed you— if anyone is above reproach, the husband of one wife, and his children are believers and not open to the charge of debauchery or insubordination. For an overseer,as God’s steward, must be above reproach. He must not be arrogant or quick-tempered or a drunkard or violent or greedy for gain, but hospitable, a lover of good, self-controlled, upright, holy, and disciplined. He must hold firm to the trustworthy word as taught, so that he may be able to give instruction in sound doctrine and also to rebuke those who contradict it.
For there are many who are insubordinate, empty talkers and deceivers, especially those of the circumcision party. They must be silenced, since they are upsetting whole families by teaching for shameful gain what they ought not to teach. One of the Cretans, a prophet of their own, said, “Cretans are always liars, evil beasts, lazy gluttons.” This testimony is true. Therefore rebuke them sharply, that they may be sound in the faith, not devoting themselves to Jewish myths and the commands of people who turn away from the truth. To the pure, all things are pure, but to the defiled and unbelieving, nothing is pure; but both their minds and their consciences are defiled. They profess to know God, but they deny him by their works. They are detestable, disobedient, unfit for any good work.
But as for you, teach what accords with sound doctrine. Older men are to be sober-minded, dignified, self-controlled, sound in faith, in love, and in steadfastness. Older women likewise are to be reverent in behavior, not slanderers or slaves to much wine. They are to teach what is good, and so train the young women to love their husbands and children, to be self-controlled, pure, working at home, kind, and submissive to their own husbands, that the word of God may not be reviled. Likewise, urge the younger men to be self-controlled. Show yourself in all respects to be a model of good works, and in your teaching show integrity, dignity, and sound speech that cannot be condemned, so that an opponent may be put to shame, having nothing evil to say about us.” (Titus 1:1–2:8)
Based on what these passages from scripture reveal, are you spiritually mature or immature?