I realize the answer to this whole question about who is a Christian will be skewed here toward the acceptance of the Trinity. But it could also be looked at as in Antioch where the term was first used to describe followers of Christ. Or it could be looked at as simply those who accept Jesus Christ as savior. Or perhaps even those who believe Jesus was indeed a significant figure whose teachings they strive to follow as they understand them in their own faith journeys.
There are two different ways of looking at it.
For purposes of canon law, a person who is validly baptised (and here the reader should insert all the criteria for a valid baptism) is accounted a “Christian.” Anyone not-baptized is accounted a non-Christian. That status applies to individual persons, not to communities.
At the same time, the Church also says that some
communities either do, or do not, practice valid baptism.
Alongside the canonical definition of Christian, we might also use the word to describe anyone who believes in Christ; not just by name or by reference, but one who actually believes in the Christ Who lived 2000 years ago, was crucified, died, and rose.
That excludes groups like Mormons who believe in an entirely different Jesus than the one who lived 2000 years ago.
It also excludes Unitarians because Unitarians believe in combining all religions together and believing that non-Christian faiths (such as Hindu and Moslem etc.) are all true and all valid. So, while they might sometimes have a practice that has all the external appearances of Baptism (water, mentioning the Trinity, etc) since they don’t believe that the God of Christianity is the only God (but just one among many equals), their ceremonies are not true baptisms.
There are those who believe in the true Jesus Christ, but who have never been baptised; at least, they believe most of what is essential in Christian belief. Obviously, they do not believe or profess the entirety of the Christian faith. We can still call these people “Christians” (and indeed, sometimes we must), even though they do not meet the definition of Christian as it is used in canon law. If the issue is a canonical one (such as a mixed marriage involving one Catholic party) then we use the canonical definition. If it’s a matter of conversation, or other matter not involving the canons, we can still use the word Christian to describe them.