According to Trinitarians, can a Christian who lacks a belief in the Trinity still live the Christian life to its full potential? In other words, can non-Trinitarians experience spiritual rebirth, regeneration, sanctification, the baptism in the Holy Spirit, the fullness of the Holy Spirit, live a Spirit-filled and Spirit-led life, etc.?
Another way to phrase the question: according to Trinitarians, if someone lacks a belief in the Trinity, will their rejection of the Trinity inevitably become a road block to their spiritual growth?
Appendix 1: On the salvation of non-Trinitarians.
My question (which is about spiritual growth) has arguably some overlap with the related question on the salvation of non-Trinitarians (which is about soteriology). Note that the latter deals with a stronger claim. Of course if non-Trinitarians cannot be saved, then they cannot be expected to achieve full spiritual maturity in Christ, because only saved people can reach that level. However, note also that the opposite direction is not necessarily true: it could be the case that someone doesn't achieve full spiritual maturity (e.g. because their spiritual growth is impaired by their rejection of the Trinity), yet God in His mercy might still grant them salvation. (This is, by the way, the reason why this question is not a dup of this one.)
That said, at the very least the subset of Trinitarians who subscribe to the Athanasian Creed must, by necessity, believe that belief in trinitarianism is essential for salvation, and, by logical implication, it follows that they must also believe that non-Trinitarians' spiritual growth is severely compromised from the get-go. However, this still doesn't answer the question in the case of Trinitarians who do not (fully) subscribe to the Athanasian Creed. For further reading on this, please see What is the biblical basis for the Athanasian Creed's statement that believing in the Trinity is necessary for salvation?
Appendix 2: On the spiritual growth of non-Trinitarians, from the non-Trinitarian perspective.
If lack of belief in the Trinity impairs the spiritual growth of non-Trinitarians, we should be able to find evidence of this across the board in non-Trinitarian denominations. One possible way to check this would be to conduct interviews with a representative sample of non-Trinitarians of different denominations. Another way is to analyze a representative sample of testimonies from dedicated non-Trinitarians and former non-Trinitarians (for a balanced view) and compare them with those of Trinitarians to see if there are any notable differences in terms of spiritual growth. And another option is to see what each non-Trinitarian faith group claims to be possible in this regard, doctrinally speaking. The last alternative is the easiest to implement of the three, so that's what I did, and below are a few examples.
As a summary, non-Trinitarians hold doctrines about spiritual growth which, at least in principle, give them no disadvantage with respect to Trinitarians. They believe to have as much access to salvation, sanctification, spiritual growth and the power of the Spirit as Trinitarians claim to have for themselves.
According to this article https://www.jw.org/en/library/magazines/watchtower-study-august-2017/fruitage-of-the-spirit-love/:
First, ask God for his spirit, which produces love. Jesus stated that Jehovah gives “holy spirit to those asking him.” (Luke 11:13) If we pray for holy spirit and endeavor to “keep walking by spirit,” our actions will become more and more loving. (Gal. 5:16) For instance, if you are an elder in the Christian congregation, you can ask for holy spirit to help you to give Scriptural counsel to others in a loving manner. Or if you are a parent, you can request that God’s spirit help you to discipline your children, not in anger, but in love.
And according to this article: https://wol.jw.org/en/wol/d/r1/lp-e/1967360
Why is this Word of God so alive, so powerful? Because it is the very essence of truth and because its penmen were filled with the spirit or active force of the Creator, Jehovah God, the omnipotent One. Its words are truly spirit and life, even as God’s Son, Jesus Christ, said. (John 6:63) Those who drink in the words of truth found in the Bible with open minds and honest hearts are bound to receive some of God’s powerful active force or spirit. Further, that Word of God imparts faith to those who receive it in good and honest hearts; a faith that enables them to do many valiant and mighty works. (Hebrews, chapter 11) More than that, the Word of God imparts strength to truth-hungry and open-minded readers by its very honesty and candor as well as by its emphasis on righteous principles. It also imparts strength for righteousness by its revelation of the all-wise, all-mighty and perfectly just Creator, the one who, above all others, is the proper object of fear.
The following comes from the article titled The Gift of Holy Spirit: The Power to be Like Christ: https://www.biblicalunitarian.com/articles/holy-spirit/the-gift-of-holy-spirit-the-power-to-be-like-christ
The subject of the Holy Spirit is one of the most misunderstood subjects in Christendom. Yet the gift of holy spirit is one of the greatest gifts God has given to mankind. Moses had it, and when he needed help administering the millions of Israelites, God took of the spirit that was upon Moses and gave it to the elders of Israel so they could rule with him. God gave His gift of holy spirit to the Judges of Israel, such as Deborah and Samuel. He gave it to kings such as David and Hezekiah. He gave it to prophets such as Elijah, Isaiah, and Daniel. John the Baptist had holy spirit from birth. Even Jesus was anointed with holy spirit before he started his ministry. Now we can walk in the power of holy spirit. This book will answer such questions as:
- What is the difference between Holy Spirit and holy spirit?
- Why is “holy spirit” sometimes referred to as “he” and other times referred to as an “it”?
- What are the manifestations (sometimes called “gifts”) of holy spirit?
- What is speaking in tongues, and why is it valuable for Christians?
- What is “slain in the spirit”?
This book shows that each Christian is sealed with God’s gift of holy spirit the very moment that he is saved. Furthermore, it shows that each Christian can manifest, outwardly show, that spirit in the nine ways spoken of in 1 Corinthians 12, including speaking in tongues. When we understand the gift of holy spirit and why God gave us such a wonderful gift, then we can take advantage of it in our lives. We can walk in the power of the holy spirit, and become more like Christ in attitude and action.
The following comes from https://www.churchofjesuschrist.org/study/scriptures/gs/sanctification?lang=eng:
The process of becoming free from sin, pure, clean, and holy through the Atonement of Jesus Christ (Moses 6:59–60).
God hath chosen you to salvation through sanctification of the Spirit, 2 Thes. 2:13.
We are sanctified through the offering of the body of Jesus, Heb. 10:10.
Jesus suffered that he might sanctify the people with his own blood, Heb. 13:12.
High priests were sanctified and their garments were washed white through the blood of the Lamb, Alma 13:10–12.
Sanctification cometh to those who yield their hearts unto God, Hel. 3:33–35.
Repent that ye may be sanctified by the reception of the Holy Ghost, 3 Ne. 27:20.
Sanctification through the grace of Jesus Christ is just and true, D&C 20:31.
Jesus came to sanctify the world, D&C 76:41.
Sanctify yourselves that your minds become single to God, D&C 88:68.