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.
Scripture teaches and if I'm not mistaken this organization most likely teaches that God has only revealed himself through the son:
Joh 1:18 No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him.
Joh 6:46 Not that any man hath seen the Father, save he which is of God, he hath seen the Father.
1Jn 4:12 No man hath seen God at any time. If we love one another, God dwelleth in us, and his love is perfected in us.
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.
It may be orthodox and traditional, but it's not biblical. Every picture of the Godhead in heaven is comprised of two individuals, the father and son:
Joh 1:1 In the beginning was the Word, and the Word was with God, and the Word was God.
Joh 1:2 The same was in the beginning with God.
Joh 1:3 All things were made by him; and without him was not any thing made that was made.
No person called "Holy Spirit" up there in heaven.
Dan 7:13 I saw in the night visions, and, behold, one like the Son of man came with the clouds of heaven, and came to the Ancient of days, and they brought him near before him.
Father and son.
Rev 22:3 And there shall be no more curse: but the throne of God and of the Lamb shall be in it; and his servants shall serve him:
No throne for the holy spirit.
Rev 22:1 And he showed me a pure river of water of life, clear as crystal, proceeding out of the throne of God and of the Lamb.
Again.
Rev 7:10 And cried with a loud voice, saying, Salvation to our God which sitteth upon the throne, and unto the Lamb.
No praise for another, separate entity called "the holy spirit".
I can give scripture after scripture that affirm the Godhead is comprised of the father and son.
Adding a third person to the Godhead obscures a great truth. Marriage is a God given picture of the type of relationship the father and son have. A perfect marriage (if one were possible) would be one where the husband and wife were completely in agreement about goals, aims, direction and ambitions. They are two separate people, but unified as one.
. 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.
I've always found biblical truth to be a more effective tool. If you feel comfortable accepting Roman Catholic tradition on the trinity instead of biblical evidence than that's your choice.