I think that many Christians have a too-simple model of a human being, and free will.
(If you are a hard determinist Calvinist, then you have many other problems,
which I will not address.)
If you think that the Holy Spirit can only dwell within a Christian who never sins,
then you cannot handle the Apostle John's statements about "if we sin, we have an
advocate with the Father, Jesus Christ the righteous."
1 My children, I am writing this to you so that you may not commit sin. But if anyone does sin, we have an Advocate with the Father, Jesus Christ the righteous one. 2 He is expiation for our sins, and not for our sins only but for those of the whole world. 3 The way we may be sure that we know him is to keep his commandments. 4 Whoever says, “I know him,” but does not keep his commandments is a liar, and the truth is not in him. 5 But whoever keeps his word, the love of God is truly perfected in him. This is the way we may know that we are in union with him: 6 whoever claims to abide in him ought to live [just] as he lived.
New American Bible, Revised Edition. (Washington, DC: The United States Conference of Catholic Bishops, 2011), 1 Jn 2:1–6.
16 Therefore, confess your sins to one another and pray for one another, that you may be healed. The fervent prayer of a righteous person is very powerful. 17 Elijah was a human being like us; yet he prayed earnestly that it might not rain, and for three years and six months it did not rain upon the land. 18 Then he prayed again, and the sky gave rain and the earth produced its fruit.
19 My brothers, if anyone among you should stray from the truth and someone bring him back, 20 he should know that whoever brings back a sinner from the error of his way will save his soul from death and will cover a multitude of sins.
New American Bible, Revised Edition. (Washington, DC: The United States Conference of Catholic Bishops, 2011), Jas 5:16–20.
Note that some in the early centuries of Christianity thought that a Christian could not sin, after converting
to Christianity, or he would be condemned at the final judgment. This is a doctrine that the Church rejected
as false.
Note that James is talking about his (Christian) disciples. He is not talking to
pagans, who might be thinking about converting to Christianity.
I would also argue that the language of "one another" does not refer to all people,
but to the People of God. This text deals with Christians, confessing their sins to
other Christians, and asking other Christians to pray for them, that God would
forgive these sins (committed after becoming a Christian).
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Assertion: The Holy Spirit dwelling in us, does not mean that God deterministically
controls our decisions.
After conversion, we are still capable of choosing to sin.
After conversion, we still need to continually choose to let God
renew our mind, so that we may know his will, and choose to do it.
There us much language in Scripture, that exhorts us to grow in
virtue and knowledge, toward the full maturity of Christ.
Theologies that are deterministic, present a ridiculous problem, in interpreting
these texts of Scripture.
16 All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness,
17 that the man of God may be complete, equipped for every good work.
The Holy Bible: English Standard Version (Wheaton, IL: Crossway Bibles, 2016), 2 Ti 3:16–17.
Why teach Christians, if the Holy Spirit does this automatically?
1 Not many of you should become teachers, my brothers, for you realize that we will be judged more strictly, 2 for we all fall short in many respects. If anyone does not fall short in speech, he is a perfect man, able to bridle his whole body also.
New American Bible, Revised Edition. (Washington, DC: The United States Conference of Catholic Bishops, 2011), Jas 3:1–2.
5 I wish to remind you, although you know all things, that [the] Lord who once saved a people from the land of Egypt later destroyed those who did not believe.
New American Bible, Revised Edition. (Washington, DC: The United States Conference of Catholic Bishops, 2011), Jud 5.
11 About this we have much to say, and it is difficult to explain, for you have become sluggish in hearing. 12 Although you should be teachers by this time, you need to have someone teach you again the basic elements of the utterances of God. You need milk, [and] not solid food. 13 Everyone who lives on milk lacks experience of the word of righteousness, for he is a child. 14 But solid food is for the mature, for those whose faculties are trained by practice to discern good and evil.
New American Bible, Revised Edition. (Washington, DC: The United States Conference of Catholic Bishops, 2011), Heb 5:11–14.
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So, back to the original question...
If all Christians are indwelt by the Holy Spirit,
why are their disagreements among Christians, as to doctrines?
There are many reasons:
-- Biblical illiteracy: Those who do not know the Bible, assert that it means all sorts of things.
-- Lack of growth in knowledge and wisdom: Christians who do not take the biblical commands
seriously, to get a renewed mind, never make a serious effort to learn. Those who hold to
anti-intellectual theologies, may stridently claim that THEY DO NOT NEED TO LEARN ANYTHING,
and stridently proclaim their ignorance, as if it were the teaching of the Holy Spirit.
-- Many younger Americans do not believe that there is any universal truth.
They think that whatever they assert about the Bible and the faith, must be true.
-- Some people, have a very flawed definition of the divine inspiration of Scripture.
They think that they are free to interpret Scripture however they want.
-- Some people are so engulfed in the "diverse culture" secular arguments,
that they claim that diverse cultures authoritatively interpret Scripture.
-- Many Christians never Bother to learn the biblical languages, and so have no way
to verify if their favorite understandings of biblical passages, are true.
There are many reasons why people who claim to be Christians, would have
very different assertions about what the Bible means.
