According to Trinitarians, can non-Trinitarians experience regeneration, sanctification and a Spirit-led life despite not believing in the Trinity?

Someone put this eloquently: "God loves just the way you are, but he also loves you too much to leave you just the way you are."

God saves searching people with inadequate knowledge of His ways and then brings them into a community (the church) that possesses adequate knowledge. Bible examples include Cornelius (to whom God sent Peter), the Ethiopian (to whom God sent Phillip), and some disciples of John the Baptist who had never heard of the baptism of the Holy Spirit (to whom God sent other apostles).

I read about a man in a Soviet prison camp who was near death. He had been raised in a completely atheist environment. All he knew about religion was that his grandmother used to say the word Jesus and it meant a lot to her. That was all he knew about Christianity: one word. So in that prison he prayed to Jesus for help. God sent another believer to him in the prison to explain the faith and that man accepted Jesus. He died soon after.

The main thing is that a truly saved person is humble, hears the Good Shepherd's voice and is led to the company of people who can correct error or supplement ignorance. How much progress they make in the faith before they die (such as in an extreme situation as above) is not as important as whether or not they trusted whatever accurate knowledge has been given to them. But in a normal life, when one has time to mature and access to the Bible, the Spirit will guide a person toward belief in the Trinity and to many other doctrines.

For example, even after many years as a believer, I still stumble upon Trinitarian passages in Scripture that I once overlooked. Anyone who loves God's Word will come upon these verses again and again in their life, and the Spirit will slowly work the truth into their life and consciousness. Here is one I found just this week:

Then Job answered and said:

2 “How you have helped him who has no power!
    How you have saved the arm that has no strength!
3 How you have counseled him who has no wisdom,
    and plentifully declared sound knowledge!
4 With whose help have you uttered words,
    and whose breath has come out from you? (Job 26:1-4)

Verse 2 is about the power of God over the physical realm (the Father).

Verse 3 is about the wisdom of God's counsel (the Holy Spirit, sent to us as our Counselor).

Verse 4 is about the spoken word (Jesus, the Word of God).

These Trinitarian distinctions pop up everywhere, and the Holy Spirit uses them to guide people into the truth, however long it takes, if they consent.

However, at a certain point, if a person refuses to accept the Trinity after it has been revealed to them, their spiritual growth will be impaired. The Parable of the Soils makes this clear.

The seed-snatching bird promotes the rejection of the seed - of Jesus as the Son of God.

The rocky soil is quenching of the Holy Spirit by fearing the world and persecution, instead of God. The Holy Spirit is associated with water and growth. Jesus said that blaspheming the Holy Spirit is the unpardonable sin; its work in our lives is essential.

The thorny soil is rejecting the focusing discipline of the Father and pursuing the riches and pleasures of this world and ignoring the call of God to follow his ways.

Thus deficiency in working in concert with any member of the Trinity will have deleterious consequences. Close cooperation requires a certain level of awareness of the distinction between the separate spheres of each.

The general outline for Spiritual Growth is:

1. Preparation (consecration, exile, being set apart)

2. Plowing (suffering)

3. Planting (the seed of God's Word by the Son of God)

4. Pouring (water by the Holy Spirit)

5. Plucking (of weeds and all distractions by the Father)

6. Producing (a Harvest of righteousness, by all three members in concert)

7. Peace

This sevenfold schema is defined in Matthew. It is the overall structure of Matthew's seven parts. In addition, smaller sections of Matthew are patterned after this sevenfold structure, sometimes paired with an instance in reverse to form a chiasm. In all, there are at least seventeen instances of this harvest pattern present in Matthew. In light of this pattern, the distinct role of each member of the Trinity is essential if one is to reap a bountiful harvest. The three aspects correspond to three parts of a person: the Word is intellectual, the Spirit is emotional (as well as the heart's desires) and the Father is physical (including habits of action, alterations to the physical world and society, such as tangible miracles).

The answer has 2 components which I will put in one statement

If it is not your fault that your knowledge of God is defective then, yes, your efforts to live by conscience (and to continue intellectually to find out more about God) then you are able to be led by the Spirit. The restriction on your side is not a restriction on God's side.

According to Trinitarians, can non-Trinitarians experience regeneration, sanctification and a Spirit-led life despite not believing in the Trinity?

The honest answer is: maybe not, correlated with the Trinitarian group's view of whether adherents of other religions can be saved (see my other answer). A Trinitarian group may also judge depending on how close the non-Trinitarian group's view is to the true Jesus (see the appendix of my other answer). A Trinitarian denomination like the Catholics emphasize the role of the sacraments which only Catholics can receive; for them the answer is most likely no. But ultimately it's for God to say on a case by case basis.

Why? It's God who gives the Holy Spirit in the first place when a Christian is born again. In all sanctification schemes it is also the Holy Spirit who plays the active role on pouring his gifts and graces to the Christian, which will be effective depending on our openness and our obedience to the Holy Spirit's promptings. He has complete freedom to choose who and in what manner He will sanctify.

Sooner or later, you will need a good theology of love for your mind to understand God's love and to know how to love better. A human being has mind, will, emotion, and built-in passions. A theology of love helps our mind to inform our will, to govern our emotion, and to understand how our passions work (which can be inflamed with grace for infused virtues) in performing an act of love.

A theology of love is the basis for a map of spiritual growth. A Trinitarian theology of love is the basis for a good and orthodox map of spiritual growth. Logically, a theology of love is inseparable from a theology of God (once you read one you'll immediately see why). Certain It's important to have a good map. Even within the boundary of Trinitarian denominations, there are various maps on offer, evident from the different kind of books they promote. These maps are usually written by theologians of spirituality.

A group will choose a few spiritual practices to teach her members. Each spiritual practice/exercise has a theology of love embedded in it as the theory behind the practice. Each spiritual practice usually has a name, a figure, a history, or even an institute behind it (such as the Catholic The Institute for Priestly Formation for promoting centuries old Ignatian Spirituality). The simpler ones are identified by a book author recommended by the group such as Richard Foster who wrote the Celebration of Discipline from within the Quaker tradition. Another well known book is Experiencing God: Knowing and Doing the Will of God written by Henry and Richard Blackaby. Other groups tend to distrust those books and will use exclusively the Bible as the guide, through small group Bible study and prayer guided by a leader vetted by the church.

Trinitarian spirituality has a rich 1800 year history since the earliest Eremitic and Cenobitic monasticism in the 3rd to 4th century AD, shortly predating even the Council of Nicaea. The benefit of going with one of the historic Trinitarian spiritualities is that it has enough time to show its fruits and has famous adherents whose lives are open to investigation. Glancing at the table of contents of a Trinitarian textbook of Christian spirituality should give you an idea of the variety (chapters 2, 5, 6, 7) and why doctrine is critical (chapters 3 & 4) to a Trinitarian spirituality:

But not all Trinitarians who have good maps practice it. Individual non-Trinitarians may end up loving God and neighbors better than some Trinitarian members! I have known very loving LDS members and I have known not-so-loving Trinitarian members including myself, sometimes! If a Trinitarian and a non-Trinitarian both have the same goodwill and openness to Holy Spirit guidance, a Trinitarian believes that he/she will go farther or easier due to a clearer, truer, and time-tested map, although I'm sure (speaking from Inclusivism perspective) God will honor them both for trying.

Conclusion: A group's responsibility is to teach, support, and recommend a spiritual practice. The group's priest/pastor and/or spiritual director can be our guide to keep us accountable and to advise us on our blind spot in our spiritual growth. Our responsibility is to practice and journey with God along with other members. The progress depends on how open we are to God's love and how committed we are to practice what the group teaches (with the assistance of the Holy Spirit). The group can help evaluate our progress but ultimately it's God himself who measures whether we are REALLY growing or only deluding ourselves.

• One must believe in the Trinity to believe in Christ:
  Summa Theologica II-II q. 2 a. 8 co.:

  It is impossible to believe explicitly in the mystery of Christ, without faith in the Trinity, since the mystery of Christ includes that the Son of God took flesh; that He renewed the world through the grace of the Holy Ghost; and again, that He was conceived by the Holy Ghost.

• Perfection consists in following (and thus believing in) Christ:

  Matt 19:21 Jesus saith to him: If thou wilt be perfect, go sell what thou hast, and give to the poor, and thou shalt have treasure in heaven: and come, follow me.

• Therefore, one must believe in the Trinity to achieve perfection.