Though I grew up as a Christian, I never understood the idea of the Trinity, and am by no means a polytheist at all, G-D forbid. I believe HaShem is a singularity, an all powerful form of sentient energy that reconciles many of the 'atheist questions' we have on G-d. For me the Shema daily prayer, stating Adonai Ehad (The LORD is One), means that for us human beings to have evolved to where we are today, there has to be a singular intelligent all-powerful force behind it to make all the delicate conditions for life. That's what my heart tells me. I did go to a Catholic primary school but I absolutely cannot understand their logic of the Trinity, more importantly I feel it is not logical to put their faith in One Man to atone for their sins. Christianity for me seems a bit idolatrous.... But I would be interested to know whether you believe in a Messiah, as I believe orthodox Jews are supposed to?
Thanks for your introduction and questions, Jane.
I should point out that there are some things that you do not need to do if you wish to explore Judaism within the Progressive framework - more than that, some of us are uncomfortable with them for a variety of reasons, in part because they are signs of 'ultra-orthodox theology'. God is not God's name - and English is not 'lashon hakodesh' (the holy language - Hebrew) - and therefore there is no need at all to write G-d. Similarly 'hashem' means 'the name'. God is not limited to a name (indeed one tradition is that there are 72 names for God!). And God does not need our protection. Saying God is not blasphemy or 'taking God's name in vain'. On the other hand, a major principle of Progressive Judaism is equality - treating all people as equals, created in the 'image of God', whether Jews or other people, whether straight or gay or 'gender-fluid' - and whether biologically male or female. We now graduate as many women as men rabbis, and a girl reads from the Torah and leads parts of the service for the Bat Mitzvah just as a boy does for the Bar Mitzvah. Women can wear kippah and tallit (the 'prayer clothing') in our synagogues just as men do. This is a very important principle, and one of the tangible differences between Progressive and Orthodox Judaism (though orthodoxy is slowly giving girls and women somewhat more say as well). But a crucial part of this is language sensitivity. Our prayer books do not say 'all men' or 'all mankind' when they obviously mean 'all people'. And similarly, we no longer use masculine terms (lord, king, master) about God, who we all know is far beyond any gender concept at all (how can women be made in the image of 'lord' or 'master'?). The tetragrammaton, God's most 'intimate', personal name (the hebrew letters YHVH) is one we cannot pronounce - because millenia ago its use was limited more and more, until only the High Priest could say it, only in the Holy of Holies, and only on one day of the year (Yom Kippur). Now we no longer have a High Priest or a Tabernacle or Temple, and have lost its pronunciation. Tradition dictates that when we see it, we instead say 'Adonai' (something like 'my Lord', though actually a plural form). Whilst tradition is important and has a vote (but famously not a veto) in the Progressive tradition, and we still read YHVH as 'Adonai' in Hebrew, we render it differently in English. Martin Buber observed that the three tenses of the verb 'To Be' in Hebrew can be made from the four hebrew letters of YHVH - that is: HaYa, was, HoVeH, is, and YiHYeH, will be. In other words, our best understanding of God's most intimate name is 'always-ness', or 'Eternal'. And we do know that God is One - a Unity - and hence we use the terms Eternal, Eternal One or Eternal God when we represent YHVH in translations.
I think this addresses much of what you say about your views of Christianity, but let me add something about a 'Messiah'. In the Progressive movement, we tend to believe or hope more for a 'Messianic time' than an individual Messiah (Mashiach). In part this is because we have been damaged too many times by claims of false Messiahs. Indeed the Rabbis teach us that if you are planting a tree (symbol of investment in the future), and they cry 'The Messiah has come, the Messiah has come', first finish planting the tree, then go and see! The Jewish test of whether the Messiah has really come, is 'is the world perfect? Have people stopped terrorising others, so each may live under their vine and fig tree, and the lion may lie down with the lamb? With that test, clearly Jesus was not the Messiah. So though we accept Jesus - probably Yehoshua or Yeshu - as a thoroughly Jewish, if perhaps radical and charismatic, teacher and healer, we reject Christ - the anointed, Messianic aspect - and indeed Christianity had to develop an entirely new theology of 'the second coming' for exactly the same reason. You could say they are still waiting for the Messiah to bring a time of perfection - and so are we! And when Menachem Mendel Shneerson (commonly known as The Lubavitcher Rebbe) died in 1994, his (ultra-orthodox Jewish) followers concluded that he was the Messiah - because not only was he so loved and saintly, but he had not appointed a successor - so he must have known that he was the Messiah! They waited by his bedside for days, and weeks, and eventually had to borrow a theology foreign to Judaism - that he would return! (Hence the 'We want Mashiach Now! movement').
But never mind them. The strongest reason, in my opinion, for not relying on an individual saviour, is that it would take the onus off us! If we seek a 'Messianic time', a 'Time of Perfection', then we should get off our daily grind, and do whatever we can to move the world in that direction. In that regard, the prayer book becomes a 'mirror of prayer', a checklist - are we matching up to God's expectations for us? Are we doing all that we have the (God-given) potential to do? And by the way, this is not really that different from Jewish tradition, as stated by Rav Isaac Kook; each of us has the potential to be the Messiah!
Hope that all helps!