do you feel it’s possible that for instance some group or groups of adventists are completely Christian, and some group of lutherans, etc. IE that no one group is completely Christian and more than one in all likelyhood constitute true Christiainity?
That is the branch theory.
Here are the passages the Spirit has led me to in confirmation of this leaning (emphasis mine):
Why do ye not understand my speech? [even] because ye cannot hear my word. John 8:43
But ye believe not, because ye are not of my sheep, as I said unto you. My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any [man] pluck them out of my hand. My Father, which gave [them] me, is greater than all; and no [man] is able to pluck [them] out of my Father's hand. I and [my] Father are one. John 10:26-30
But as many as received him, to them gave he power to become the sons of God, [even] to them that believe on his name: Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God. - John 1:12-13
Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and [of] the Spirit, he cannot enter into the kingdom of God. That which is born of the flesh is flesh; and that which is born of the Spirit is spirit. Marvel not that I said unto thee, Ye must be born again. - John 3:5-7
There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus. Gal 3:28
And I, brethren, could not speak unto you as unto spiritual, but as unto carnal, [even] as unto babes in Christ. I have fed you with milk, and not with meat: for hitherto ye were not able [to bear it], neither yet now are ye able. For ye are yet carnal: for whereas [there is] among you envying, and strife, and divisions, are ye not carnal, and walk as men? For while one saith, I am of Paul; and another, I [am] of Apollos; are ye not carnal? Who then is Paul, and who [is] Apollos, but ministers by whom ye believed, even as the Lord gave to every man? I have planted, Apollos watered; but God gave the increase. So then neither is he that planteth any thing, neither he that watereth; but God that giveth the increase. - I Cor 3:1-7
I will however challenge any Christian brother or sister if I see in their translations, doctrines or traditions an instance where there is a failing to give God the glory.