This article is intended to give some guidance to those who have recently become persuaded of a Unitarian view of God. Having gone through this myself, I know that a big theological change like this can be a time of upheaval. So, the goal of this article is to hopefully help others who are in the midst of the change to navigate the situation that will likely unfold for them, especially if they had been involved in religious community prior to their change in belief.
Once a person has become persuaded that Unitarianism is true, they have already "left Trinitarianism" in a technical sense, even if they do not tell anyone. But, keeping one's beliefs a secret is difficult for those who regard their beliefs to be an important part of their life, it is Biblically discouraged (Matthew 5:14-16), and it will feel disingenuous. However, it is also true that telling one's family, or church friends, is extremely difficult, because the reactions can be very volatile, especially if one is in a more fundamentalist community.
When I converted to Unitarianism, I had a decently large selection of Christian friends, all of whom were Trinitarian. I knew them well enough to be able to predict how they would respond to me converting. But, in my case, the stakes were relatively low, as I was not married when I converted, nor did I attend a church.
The most important single task in this process for those who are married is to preserve their marriage. Churches can be changed, new friends can be made, and extended family can cool off over time. But separating from one's spouse is something that should be avoided at nearly any cost.
For those who are married to conservative Christian spouses who they can tell are potentially not going to take this news well, I would strongly advise against bringing up the topic in any sort of casual manner. I would not mention the fact that I am listening to or reading Unitarians, I would not make a "suggestion" at a Bible study, or any other sort of quick and sudden "reveal". Instead, I would write a very careful, well-thought-out letter to them, explaining what you believe, and why. It should take a few weeks to write. It should be totally devoid of vitriol, sarcasm, or mocking. It should be something that you may have to rewrite a few times. It should be totally sincere, understanding, and tolerant. You should be able to explain, using your own words, why exactly you came to the conclusion that Unitarianism is true. Avoid pasting links to articles or videos. Then, give the letter to them, and have them read it and think it over. I believe that using a letter is going to do a lot to prevent the knee-jerk, emotional reaction that you can expect with a normal conversation, in which you will be trying to remember all your points, will potentially be nervous, etc.
Those who have close Christian friends can do the same for friends. I think a good general rule for telling others is to avoid being curt, or suddenly "revealing" it in a nonchalant or haphazard way. Doing so is likely to illicit the knee-jerk, emotional response, without much thought.
In my case, I wrote a "letter" to my friends explaining exactly why I changed my mind, in the form of a webpage that was private, after I had already told them that I was a Unitarian. In hindsight, I would have waited to tell them, and would have just given them the letter in the first place, and asked for them to respond in a few days. I also would have taken more time, and done more study, before writing my letter.
If you have taken steps to present your conversion thoughtfully, then you have already done your part. How others react at that point is dependent on them. If your Christian friends are tolerant, then you may have very little trouble at all. If, on the other hand, you are in an extremely fundamentalist context, then this is going to be a pretty difficult time. People that you love may call you non-Christian. They may try to expose you to other Christian friends and family, or disown you as a friend, or, simply ignore you from now on.
Does someone responding in this way mean they were never "really" your friend, or never a "true friend"? Not necessarily. It could be that they care deeply for you, and view this as an extremely damaging departure on your end, which may, in their theology, potentially damn you. They may simply not know how to constructively respond, or may be touched by something you said, and lash out as a defense mechanism. People are complicated, and someone reacting poorly, especially at first, does not necessarily mean that they are a bad person, or were never a "true friend", etc. It does not necessarily mean that they won't be a friend in the future. Your job is to avoid sarcasm, passive-aggressiveness, or being belittling when you respond to them, regardless of what they say. Aim to leave the door open for reconciliation, and do not repay evil for evil.
In my own circumstance, when I haphazardly revealed that I was a Unitarian in various settings, some of my friends abandoned me and called me non-Christian, others did not. You will probably be able to tell ahead of time who is more disposed to responding negatively. I believe that it is worth the extra effort to take special care with such people, regardless of how they wind up responding in the end. You became friends for a reason, and so you shouldn't feel eager to just burn bridges and leave people behind, if it can be avoided.
When making a major theological change, a very common thought for one to have is, "If I have been wrong about something this big for so long, what else am I wrong about?". When a person becomes a Unitarian after having been Trinitarian, they are acknowledging that they have believed something fundamentally wrong about the nature of God, potentially for years, or decades. Having believed something false for so long on such a fundamental doctrine can make it feel like one's base has been swept out from under them, and that can be disconcerting.
Because people will be coming from such a variety of theological backgrounds, it is impossible to give a one-size-fits-all answer to this concern. I believe that the best advice that can be given is for one to take their time when re-examining any of their other beliefs. It is good to be willing to change a belief in light of new information. However, give each study the time it deserves, and do not feel like you need to subscribe wholesale to an entire new theology, because you are looking for somewhere stable to plant yourself. It is highly unlikely that everything that your Trinitarian church taught you about the Bible and Christianity is wrong. If nothing else, the moral precepts taught in most Trinitarian churches - loving God and neighbor, with all that entails - will probably carry over very closely.
Another concern for a new Unitarian is that they have found themselves in a theological minority. Most Christian churches in the world believe differently from them on this major issue, and many of them even anathematize them and their position (at least on paper - which many laymen in the congregations may actually have little regard for, or even knowledge of). Suddenly, one's social opportunities - finding Christian friends, a church, a spouse - are constrained, and pretty drastically.
Many towns in the United States - a very theologically tolerant and diverse nation - do not have any Unitarian congregations. Outside of the United States, it is almost certainly even more difficult for the average person to find a church to attend. This drives many Unitarians to seek online fellowship. Still, it can feel socially isolating to belong to this tradition, especially for a person who takes religion seriously, and makes it a large part of their life. It is simply true that being a theological minority is more lonely than belonging to the majority position.
My only advice in this regard is to try to cultivate an "I will make it work" attitude. A Unitarian should try to avoid feeling sorry for themselves, and try to focus on what social opportunities do exist for them, especially online first, as there are many established communities there already.
Navigating a large shift in one's theology can be difficult, especially when switching to a minority theological position. Hopefully this article has aided in giving an outline, and establishing some general principles which will ease the transition for those undergoing it. In all things, humility and patience will go a long way. Resist the temptation to be bitter, or to repay evil for evil. Be content in knowing that you have the truth on this important topic, and trust that God will work things for good in your life (Romans 8:28).