Why do Christians care so much about what you believe?

This seems like a rhetorical question and it would probably be better if it was broken up into smaller questions like:

• Why does it matter if you believe that the Holy Spirit proceeds from both the Father and the Son or just from the Father?
• Why does it matter if you believe about Jesus's nature (miaphysite vs monophysite vs etc.)?
• What difference does it make whether you believe Christ has two natures or one?

Regardless, the short answer is: Because it matters and maybe also Becasuse people are proud, while knowing nothing -1 Timothy 6:4

Whenever there are 2 conflicting theories, only one or none of them can be the truth. And the truth is important. Even a small deviation from the truth can have mayor consequenses(ripple effect).

But I am afraid that as the serpent deceived Eve by his cunning, your thoughts will be led astray from a sincere and pure devotion to Christ. -2 Corinthians 11:3

And no wonder, for even Satan disguises himself as an angel of light.  -2 Corinthians 11:14

He is proud, knowing nothing, but doting about questions and strifes of words, whereof cometh envy, strife, railings, evil surmisings,  Perverse disputings of men of corrupt minds, and destitute of the truth, supposing that gain is godliness: from such withdraw thyself. -1 Timothy 6:4,5

Let’s go back to the very first doctrinal error in history. When debating the serpent, Eve said that God had said you must not eat of the tree of knowledge of good and evil nor must you touch it, or you will die. She made up the “do not touch” part. Once Adam saw her holding the fruit without suffering harm, it was like one teen telling another, “I told you the ice is thick enough.”

The smallest error can lead to deadly consequences. That is why God gave us the third commandment against taking the Lord’s name in vain. Anyone who says something not authorized by God is appropriating God’s authority for themself, thus taking the place of God.

If the Bible is logical and true and free from contradictions, any false statement must, by some chain of logic, lead to a contradiction, barring the undecidable statements hypothesized by Gödel. Any edifice of thought built from such a statement as it grows over time must eventually encroach on vital truths, choking them out. Eventually the core of the gospel is attacked and you have a cult. At the very least you have an apologetic problem used by opponents to discredit Christianity as illogical. Small errors inevitably grow.

Christians care very much about who you believe. Is your belief in an utterly truthful source, which you can put trust in for your everlasting security, or is your belief in whatever current trends and social media stories are doing the rounds? As this famous first century Christian author wrote:

"I know whom I have believed, and am persuaded that he is able to keep that which I have committed unto him against that day." 2 Timothy 1:12 A.V.

He spoke of his belief in the resurrected Jesus Christ, who had appeared to him (after his resurrection). The reason Paul's life was transformed by that encounter was that he discovered Christ to be Truth, Light, and Life, in whom alone salvation is to be found. That is why Christians who are concerned about matters of belief take Jesus' every word seriously, as recorded in the Bible. That is why the Bible puts such stress on knowing exactly who this Jesus is - the Son of God, not merely a good man who set a fine example. Following the resurrected Jesus Christ is what Christianity is all about, but if you are following a religious system, or think that religious ritual is important, the chances are you are not following the Christ of the Bible.

Related to this is the problem of only having an academic interest in Christianity and the Bible. That's guaranteed to mire people down in pedantic points and to miss the point of why Jesus was crucified then resurrected, and why he had to be fully God and fully Man. This is what a friend of mine recently wrote:

"Had [Christ] been merely human, he could not have been sinless. Had He not been human He could not have died. Had He not died, He could not have conquered death by His sinless life. Atonement could only be attained by a sinless Man who was also God!

...Arguments about the authority and cultural relativity of the scriptures are simply smoke and mirrors. If their authorship is merely human, the Bible is fallible. If 'men spoke from God as they were carried along by the Holy Spirit' (2 Peter 1:21), then the Author is God Himself and we are fools to be questioning a text that ought rather to be questioning us. The touchstone of credibility and reliability is the character of the Divine Author, not the flaws of His mortal secretaries.

Finite study (we are not capable of better) without reliance on the infinite Mind of God is a completely illogical exercise - which must lead to illogical deductions." David Andrews, p. 5 of The Sword mag. Jan/Feb 2023, article "The Substitute".

This answer shows the chasm between unbelievers like yourself, and Christians who believe passionately in the person of the resurrected Jesus Christ. The question is, who do you believe, and do you know who this One really is?

It will matter what one thought (and believed) about God on the day when the universe (and time itself) ends . . . . or on the day when one's heart stops beating and one discovers what death really is. Until one learns to fear the future (an eternal future) it is easy to mock and question and contradict .

The Christian faith begins with repentance, before ever faith forms.

The Christian faith begins with the fear of God and the conviction of one's own sinful acts and one's own sinful nature.

Until these things take hold of the soul, all else is just theoretical arguments.

But once they do, the soul cannot rest until God is found, in Christ, the only possible answer to one's own human condition and the only possible answer to a chaotic and doomed world.

Christians care about what you believe because at root Christianity is about your faith, not what you do.

In most other religions, especially in the Judaism that Christianity came out of, your adherence to your religion is largely judged by what you do. You are a good Jew if you keep Shabbat and avoid prohibited food. You are a good Muslim if you pray the daily prayers, and refrain from prohibited food and drink. ¹ You are also required to believe things, but the practices are an important part.

By contrast Jesus explicitly taught that what you eat and drink is not important, and that there was no need to follow the ritual practices of Judaism (the religion his followers mostly adhered to). Instead he very much emphasized "faith", specifically faith in himself, as the defining mark of his followers, with an unspecific "love" a close second. He commanded almost no specific acts, ceremonies, rituals or practices for his followers to do. That's not to say Jesus doesn't expect high standards of behaviour from his followers, but he is less specific about exactly what that behaviour needs to be.

Faith is closely related to belief (while not being exactly the same thing). Therefore the defining mark of a Christian becomes what you believe, not what you do. It is your belief, your faith, that makes you a Christian. Therefore it is unsurprising that Christians care a lot about the nature of that belief.

Belief about the nature of Jesus is particularly important, because it is faith in him that defines the Christian. If you say you believe in Jesus, but believe that he is wholly different from what he actually is, then are you really believing in him?

This can of course be carried to excess, and does lead to some Christians getting into furious arguments about whether you should believe that the rapture of the faithful will take place before or after the Tribulation in the End times (for example - look it up if you want more detail).

Notes

1. I'm aware I'm oversimplifying here, but the principle is true.