There are in all likelihood, hidden, in various Christian communities, those who affirm that God is One, and that Jesus is His Son and Messiah, and keep it that simple. There are also quite a few Jewish believers in Christ who continue to believe that God is One, and that Jesus is His promissed Servant, Son, and Shiloh. I have read some Eastern Orthodox sources which while affirming the Father, Son, and Holy Spirit, place a specific and essential emphasis on the Godhood of the Father. The earliest Christians had a variety of views and it seems that a diversity of views were far more common in the first 200 years, so long as Christological essentials described in the Gospels were not denied. Martyrdom, charity, purity, renunciation, and shunning idolatry seem like the likeliest concerns early on, prior to speculation regarding the nature of Christ becoming more nuanced and divisive.
It seems that some kind of Adoptionism was even fairly wide spead, though apparently not anything like a majority view. Michael Servetus rightly speculated that belief in a Triune Godhead made the Gospel far less comprehensible to both Jews and Muslims, who may have been far readier to understand and hear the Gospel had Christian theology been presented to them in a simpler and more Apostolic manner. I know there must be some who have also concluded the there is One God and that Jesus is His Son and Messiah also call upon the Son just as they do the Father, and praise Him, too.
I'm not intending this to become a debate on the Unity or Triunity of God. I have met Unitarian Christians who have been awful witnesses, and Trintarian Christians who have been awful witnesses. Our theology is no guarantee of our being fruitful, kindly, or decent followers of Christ. When did Christianity become less about renunciation, simplicity, charity, and discipleship and more about creeds, academia, anathemas, and pogroms? Had all of us kept things basic and charitable, perhaps we wouldn't have had the Great Schism, and Luther would have seen no reason to "Reform" anything. But as we can't turn back time, we can at least give up everything to the One who is the same yesterday, today, and forever, and live for Him, in Him, and because of Him.