Lots of people know that for Christians Pride is the worst sin. This is distinctive to Christianity - where it has a particular meaning of rejecting the authority of God, the validity of God's creation, the Goodness of God.
If you don't understand Pride, or disagree with it being a sin, then you aren't a Christian.
(Of course, you may simply disagree about whether a particular thing is an instance of Pride and therefore a sin. But a Christian cannot deny that Pride is indeed a sin.
But you don't need to live consistently without Pride! Christianity is about 1. acknowledging that Pride is a sin, and 2. repentance of the sin of Pride in oneself when it happens; Christianity is not conditional on the (impossible?) achievement of living entirely without Pride. (Christ came to save sinners, not perfect men.)
Some sins are almost universal, shared between religions and no religion, and seem 'natural' - but Pride is not one of these. Pride is distinctive to Christianity, and that Pride is a sin is known as a consequence of Christian revelation - it is not a product of instinct nor a result of logical analysis.
This is a living, active dispute. Many campaigns and organizations are structured around the goal of stimulating and sustaining Pride in some group of persons. Typically, they will officially define Pride in non-sinful terms such as 'self-respect', or 'love for' or loyalty; but equally typically they will sooner or later advocate sinful Pride - including the subversion, rejection or inversion of the Christian understanding of other sins.
The sin of Pride usually leads swiftly onto resentment and hatred - and this is a reliable way of detecting sinful Pride, and discriminating it from benign or neutral self-respect, love or loyalty. Non-Christians often argue that Pride is necessary - and indeed it does seem to be psychologically necessary to non-Christians, or at least mostly-so.
Most religions regard Pride as a good thing, not a sin - so long as it has the appropriate subject matter. For modern atheist/ agnostics, Pride may be the core of their being, the thing that keeps them motivated and active.
I would say that this was largely the case for my pre-Christian self: that Pride was what enabled me to stand against the social consensus, what gave me strength to do my thing. Pride was the fuel and Pride provided my direction: thus I would not have been willing to agree that Pride was a sin, and certainly not the worst sin.
This is perfectly understandable and maybe inevitable - if you are not a Christian.
So, the attitude to Pride is indeed a cleavage line between Christians and non-Christians. Pride can only be seen as the evil it is, when a person acknowledges that the values of the universe do not arise from himself - but are established by our Heavenly Father who is 1. the creator, 2. Good, and who 3. loves us - we being his children.
So the prevailing 'rules and laws', the order of the universe... this is not arbitrary, nor does it come from ourselves - these things are true, objective, external (even when our knowledge of them is imprecise and prone to error) - and the 'set-up' of creation is essentially benign and made for our ultimate benefit (individually and as mankind).
Because God is the creator, Good and our Father - God has legitimate authority over us. Or, to put it another way, since God created order - there is no ordered Good except within God's creation. If we reject God, then we, Pridefully, are setting ourselves up as a rival God - yet we have no created order to dwell within. Our position is therefore oppositional to the created, ordered Good.
Or else, insofar as it is not merely oppositional, in Pride we come to dwell in a microscopic subjective universe of our own self-will - perhaps trying to persuade others to subordinate themselves to our subjective micro-universe. We make ourselves God of a tiny world where nothing has created by us, but only co-opted from God's universe and subjected to our personal interpretation.
This Pride-full, micro-subjective universe situation is not impossible, it is not even irrational - but it is anti-God, anti-Christian, a self-exile outwith the bounds of Christianity.
Christians call it Hell - it is in fact plural: a group of multiple mutually-self-isolated mini Hells - but clearly some people prefer it and choose it. That choice is the consequence of ultimate Pride.
Pride is a rejection of the basic fact that our shared reality comes from our Christian God - and the demand to define reality from within ourselves - which is why it is incompatible with Christianity.