The protestant churches are, mostly, corporate churches now. The ecumenical movement (a RCC thing) exists to bring these churches back to the fold under a common religious banner. Even the Seventh Day Adventist church has been corrupted by the ecumenical movement.

The ecumenical movement is directly contrary to Christ's direction. We are commanded to come out of corruption and be separate.

Other churches have begun adopting certain aspects of Catholic doctrine. The big examples are the Trinity, Sunday worship (which most never left in the first place), advocacy for Sunday laws, the substitution of church tradition over Biblical law, celebacy in the priest hood (Paul specifically said priests should be married in 1 Timothy), belief that judgement comes in the moment of death (looking down from Heaven or roasting in Hell), etc.

The Seventh Day Adventist church accepted the Trinity doctrine back in 1980. They try to pretend it isn't the same as the Catholic doctrine and if you read the fine print, it isn't, but in a few decades it will be indistinguishable.

The Bible is pretty clear on the Father, Son, and Holy Spirit. In the Gospel Christ says he will return to His apostles in Spirit and that He and His Father will be among them in Spirit. This is the Holy Spirit. The Holy Spirit is Christ/God. It isn't a third being. The RCC teaches of a triune godhead which aligns with Babylonian, Egyptian, and Theban teaching, all of which have a sungod somewhere in their theology. In Babylonian, that's Tammuz. In Egyptian it is Amun-Ra (one step up from their trinity). In Theban it is Amun. What day did the Catholics "move the sanctity of the Sabbath" to? Sunday. Sunday was so-called because it is the day the Romans used to worship their sungod. What does Lucifer mean? Light-bringer.

Exekiel 8:13-16.

13 He said also unto me, Turn thee yet again, and thou shalt see greater abominations that they do. 14 Then he brought me to the door of the gate of the Lord’s house which was toward the north; and, behold, there sat women weeping for Tammuz.

15 Then said he unto me, Hast thou seen this, O son of man? turn thee yet again, and thou shalt see greater abominations than these. 16 And he brought me into the inner court of the Lord’s house, and, behold, at the door of the temple of the Lord, between the porch and the altar, were about five and twenty men, with their backs toward the temple of the Lord, and their faces toward the east; and they worshipped the sun toward the east.