I don't know much about the organization other than I enjoyed this article. I do however agree with many of their views. I consider myself a first century Christian. Those in biblical times hadn't "developed" a notion of the trinity. That's an evolved viewpoint hashed out by men over the centuries and agreed upon by tradition. Biblical Christians believed the father and son to be divine persons in the Godhead and considered the Holy spirit to be their intercession and presence in our reality and lives. I don't believe that's modalism.
Sabbath keeping was the norm in biblical times for Christians as was observing God's holy days. Days such as Christmas and Easter are not biblical holy days. They too are evolved traditions derived from what is now called the Roman church.
I am not ashamed of embracing and doing the same things my Lord Jesus Christ and his followers taught and did and I'll gladly wear the label of heretic if that's the cost.
Modalism is where God is behind the scenes, and shows himself sometimes as Father, sometimes as Son, sometimes as Holy Spirit. Since his real person is though God...behind the scenes, we never encounter him personally—since we only know him through those roles. I’m not saying you are believing that, however, the author of the website and the organization/church he represents seems to be teaching that.
While the word trinity itself is not found in scripture, all of its concepts are. The Father is God, Jesus is God, the Holy Spirit is God. At the same time, the Father is not Jesus, the Father is not the Holy Spirit, and Jesus is not the Holy Spirit. Distinct real persons (not merely roles) yet each one one in essence. While describing it that way comes from tradition, none-the-less it is fully orthodox and biblical—and has been the formula since at least Athanasius, before Nicea. Show me Christians who compromise on this, and I’ll show you Christians becoming a cult—it is that important.
As far as holidays, in importance, its a mere distraction (though I do believe there is biblical evidence the 1st Century churches worshiped on Sunday), and not important, according to Col 2:16. I would never cry “heretic” or break with someone over the observation of holidays. On the nature of God, the Trinity, definitely....and besides, it’s a point of commonality with Roman Catholics (they’ve always been orthodox on the Trinity)—so to convince them, one should use orthodox sources.