They do all agree, the Catholics, Baptists, Greek Orthodox, Methodists, Lutherans, Assembly of God and on and on all agree that Mormonism is a different religion than Christianity.
Here is the Catholic view in short version.
Q: I read recently that the Catholic Church had rejected Mormon baptism, since their view of Christ and the Trinity is so unusual. But I have to ask: Are Mormons considered separated brothers and sisters? While their views are strange to say the least, they are still separated, and we should reach out to them. If we view them as something other than separated, doesn't that exclude ecumenism? I know that many view them as a cult, but aren't cult members separated as well?
A: The reason Mormons are not considered separated brethren is not because they aren't "separated" from the Church-they are-but they aren't "brethren" in the sense required.
The phrase separated brethren refers to those who, though separated from full communion with the Catholic Church, have been justified through baptism and are thus brethren in Christ. The Decree on Ecumenism (Unitatis Redintegratio) of Vatican II teaches that "all who have been justified by faith in baptism are members of Christ's body, and have a right to be called Christian, and so are correctly accepted as brothers by the children of the Catholic Church."
Because Mormonism is polytheistic and rejects the Trinity, Mormon baptism is not valid, and Mormons are not considered separated brethren. For the same reason, outreach to them, while certainly a good thing, is not ecumenism, though it can include dialogue and social cooperation as well as efforts to evangelize them.
Roman Catholics PRAY TO saints.... THAT is believing in multiple gods to some, yet I would not say they are not Christians.
Look, I am not Mormon, I disagree with a huge portion of their beliefs. Here is the question...
WHAT IS A CHRISTIAN?
This question assumes that there is one and only one correct definition of the term “Christian.” However, depending upon your understanding of the nature of truth, many definitions may be “true” to various groups:
To conservative Protestants, a Christian is often defined according to their salvation status. Their definition is “true” to them, because it agrees with some of their foundational beliefs: that the Bible is inerrant, that salvation is by grace, and that one must be “born-again” to be saved and avoid eternal punishment in Hell.
To Roman Catholics, a Christian is often defined according to their baptism status. Their definition is “true” to them, because it agrees with their fundamental beliefs about the nature of sacrements, their understanding of the Bible, the declarations of many Church Councils, the statements of many popes, and their church’s tradition.
To many in the very early Christian movement, a Christian was defined as a person who was baptized and proclaimed “Jesus is Lord.” Their definition was “true” to them because it agreed with their understanding of their religious belief at a time when the Christian Scriptures (New Testament) had not yet been written and assembled.
And so on, with other faith groups.
Each group has their own definition of “Christian” which agrees with their own beliefs about the nature of Jesus, God, church tradition, written text, evolved theology, the cultures in which they are implanted, etc. There appears to be no way to compromise on a single definition that is acceptable to all. One apparently cannot call on a higher power to resolve the problem, because there seems to be no way to assess the will of God on such matters. If there were such a method, then different definitions would have been harmonized centuries ago. People would simply have prayed to God and asked Him to define what a Christian is. Then, a consensus would exist today on the true meaning of the word “Christian.”
There is no consensus on what the “correct” definition of “Christian” is. There is only a near consensus within individual faith groups. Therefore questions like “Are you a Christian?” or “How many Christians are there in the U.S.” are only meaningful:
Within a single denomination, or among a group of similarly-minded denominations.