Is error in doctrine always sin?

[Sam]

What I consider essential

--Belief in the deity of Christ
Jesus said, "If ye believe not that I AM, ye shall die in your sins" John 8:24.
Both trinitarians and oneness believe in this. We believe that YHWH of the OT came to us in the person of Jesus.

--Belief in the humanity of Jesus, that He is the Anointed One (Christ) come in flesh.
2 This is how we know if they have the Spirit of God: If a person claiming to be a prophet acknowledges that Jesus Christ came in a real body, that person has the Spirit of God. 3 But if someone claims to be a prophet and does not acknowledge the truth about Jesus, that person is not from God. Such a person has the spirit of the Antichrist, which you heard is coming into the world and indeed is already here. 1 John 4:2-3
Both trinitarians and oneness believe this

--Belief that Jesus died, was buried, and rose again
1 Corinthians 15:1-4 defines the Gospel (Good News) as preached by the Apostle Paul. There he says:
1 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
2 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
3 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
4 And that he was buried, and that he rose again the third day according to the scriptures:

He stated to the assemblies in Galatia that if anyone (human or angel) preached any Gospel other than the death, burial, and resurrection of Jesus, he cursed (damned) that person. See Galatians 1:6-8

I would consider the above to be essential.

Among nonessentials I would list
--the amount of water used in baptism
--the words spoken in baptism
--the words we use to describe our particular understanding of the Deity and humanity of Jesus Christ
--whether wine or grape juice is used in communion
--whether leavened or unleavened bread are to be used in communion
--if footwashing is to be practiced literally in 2010
--if the holy kiss is to be practiced literally in 2010
--if the term "Reverend" can be used for humans
--animal sacrifices, levirate marriage, circumcision, and tithing which were all practiced before the law of Moses, incorporated into the law of Moses, and done away with under the New Covenant
--observance of any sabbath, feast day, or Lord's day
--preterism, dispensationalism,
--intercession of the (dead) saints
--devotion to Mary the Theotokos
--soul sleep or consciousness after death
--clothing rules and hair length rules
--jewelry, cosmetics, deodorant,
--the use of medicine, doctors, hospitals, vitamins, food supplements
--caffeine, nicotine, sugar, alcohol, chocolate, etc