Ready to Harvest is a YouTube channel that seeks to as objectively as possible discus the differences between different Christian Denominations. But they also did a video on the question of defining what it means to be a Christian.
He doesn't seek to definitely answer the question in the video, but there is a general sense that he feels "Anyone who follows Jesus" is too lose a definition, that at least within the Church there needs to be allowance for denying the status of Christian to Christians who are sufficiently heterodox. The Problem is Scripture is more interested in what absolutely proves someone is a True Believer then insisting anything can prove someone is not.
First verse I want to bring up is 1 Corinthians 12:3.
"Wherefore I give you to understand, that no human speaking by the Spirit of God calleth Jesus accursed: and that no human can say that Jesus is the Lord, but by the Holy Spirit."
The last part of that verse is a pretty bold statement, Paul doesn't even consider it hypothetically possible for a person to say Jesus is The Lord if they don't have The Holy Spirit.
Now I am inclined to believe that every time The New Testament says "The Lord" it's meant to be understood as a reference to YHWH, in The Hebrew Bible Adonai is a title only for YHWH, and by NT times the Jewish habit of refusing to actually say the Name but say Adonai/The Lord instead was already being established. So that can make what it means to say Jesus is The Lord a bit more specific then a plain English reading of this verse at first implies and would prevent this verse from applying to Marcionites or JWs.
But regardless the implication is any person saying that must have The Holy Spirit no matter what else they are wrong on and no matter how bad their behavior is.
A Second verse to bring up is 1 John 4:15.
"Whosoever shall confess that Jesus is the Son of God, God dwelleth in him, and he in God."
Again, anyone confessing that Jesus is The Son of God is a true regenerated Believer, no matter what other doctrines they are wrong on or what they actually do. Later 1st John 5:4-5 says that anyone who believes Jesus is The Son of God overcomes The World because they are Born of God.
The importance of that being the defining Confession is backed up by a number of passages, it's the first verse of Mark's Gospel and then what the Centurion proclaims in 15:39, it's what Peter confesses before he's given the Keys to the Kingdom in Matthew 16:19 (and again in John 6:69), it's Martha's confession in John 11:27, and what John 20:31 says we are given Life by believing, and it's the only thing the Ethiopian Eunuch has to confess before being Baptized in Acts 8:37.
Okay most of those verses also include believing Jesus is The Christ, Christ being the Greek equivalent of Messiah. In the modern world what it means to believe Jesus is The Messiah has become a bit watered down as now any kind of prophesied Hero figure can be called a "Messianic Archetype" even if not from an Abrahamic culture, but back then that Language for a promised Savior was uniquely Jewish. Believing Jesus is The Christ meant believing He was the instrument of the fulfilment of YHWH's promises to Israel, so again Marcionites and Gnostics putting Jesus in conflict with YHWH don't actually believe Jesus is The Christ no matter how much they appropriate that Language.
But let's get back to the Son of God issue.
I do believe this requires believing in Jesus as The Son of God in a very unique distinct sense separate from how all Believers and ultimately all people are God's children. In Brigahmite Mormonism Jesus is the Son of God only in being the Firstborn of humanity.
I believe the statement that Jesus is God is Biblically supported, Colossians 2:9 says Jesus was the fulness of The Godhead made Bodily giving support even specifically to Homousianism, and in the opening verses of the Fourth Gospel The Word fits the modern definition of capital G God, the Uncreated Creator of the Universe. I am also a Trinitarian having a post on this Blog about how even The Hebrew Bible is Trinitarian.
But Confessionally and Creedily speaking Jesus as The Son of God is the Biblical emphasis. Romans was the only Epistle Paul wrote to a community he hadn't known in person yet so it features his personal statement of Faith at the beginning and it never directly calls Jesus God but stressed Jesus as The Son of God. The Rule of Faith documented by Irenaeus and Tertullian and the Old Roman Symbol each emphasize Jesus as The Son of God not Jesus as God.
So I kind of am uncomfortable with how often modern Statements of Faith put all the emphasis on Jesus being God with His Sonship relegated to "The Son" as a title.
In 1 John and 2 John three things are defined as being Antichrist, denying that Jesus is The Christ, denying the relationship of the Father and the Son and denying Jesus came "in the Flesh". The Qur'an is hostile to God having a Son in any sense, it utterly rejects the idea of God as a Father, so that's why Muslims can't count as Christians.
I have decided not to make a Bodily General Resurrection or Universal Salvation requirements for truly qualifying as a Christian because of these Biblical observations, even though it is my opinion that those two doctrines are the definition of The Gospel based on 1 Corinthians 15 and thus definitely more important then any of the nuanced Greek semantical issues debated at the seven Ecumenical Councils or for that matter what the Protestant Reformation was originally about.
Now one might make an objection that a given group "doesn't mean the same thing we mean by Son of God", and they could be a valid argument if the Arians or Unitarians were the ones blatantly twisting the very definitions of those words. But the truth is we Trinitarians are the ones departing from what it normally means to call someone the Son of someone else by believing The Son and The Father are Co-Eternal.
Now my way of addressing that criticism of Trinitarians involving my belief that Jesus became The Son during the Incarnation. But most mainline Christians don't like that argument because they're bought into the Platonic Heresy of Divine Immutability.
So yes any form of Nicene Christianity is real Christianity but also plenty of Non Nicenes could be as well.
