How is it that everyone's cocksure of their own religion or spiritual path?

There are genuine paths in all religions that lead to the Truth. There are, at the same time many blind alleys, again, in ALL religions, that seem to digress from the Truth, and therefore, confuse and confound. Many ways lead to New York; some start from New Delhi, some from Berlin, some others from London. The idea is to reach New York. If your map is correct, you will definitely reach your destination even though your path looks dramatically different from another person's starting from a different place. For example, Hinduism is not all about pleasing gods for material boons, as you seem to put it, but there are umpteen strands that stress pure rationality, like Advaita and Trika Shaivism. So, let us not compare. Let us stick to what suits us and move ahead towards the Truth.

Most people have convictions, they go on faith. Those who go on faith aren't sure, they have not gone beyond doubt and they do not have verified confidence.

You can be as convinced as you can be but that isn't same as knowing & seeing.

One who knows & sees will deny having faith and convictions.

I have heard that on one occasion the Blessed One was staying in Savatthi, at the Eastern Gatehouse. There he addressed Ven. Sariputta: "Sariputta, do you take it on conviction that the faculty of conviction, when developed & pursued, gains a footing in the Deathless, has the Deathless as its goal & consummation? Do you take it on conviction that the faculty of persistence... mindfulness... concentration... discernment, when developed & pursued, gains a footing in the Deathless, has the Deathless as its goal & consummation?"

"Lord, it's not that I take it on conviction in the Blessed One that the faculty of conviction... persistence... mindfulness... concentration... discernment, when developed & pursued, gains a footing in the Deathless, has the Deathless as its goal & consummation. Those who have not known, seen, penetrated, realized, or attained it by means of discernment would have to take it on conviction in others that the faculty of conviction... persistence... mindfulness... concentration... discernment, when developed & pursued, gains a footing in the Deathless, has the Deathless as its goal & consummation; whereas those who have known, seen, penetrated, realized, & attained it by means of discernment would have no doubt or uncertainty that the faculty of conviction... persistence... mindfulness... concentration... discernment, when developed & pursued, gains a footing in the Deathless, has the Deathless as its goal & consummation. And as for me, I have known, seen, penetrated, realized, & attained it by means of discernment. I have no doubt or uncertainty that the faculty of conviction... persistence... mindfulness... concentration... discernment, when developed & pursued, gains a footing in the Deathless, has the Deathless as its goal & consummation."

"Excellent, Sariputta. Excellent. Those who have not known, seen, penetrated, realized, or attained it by means of discernment would have to take it on conviction in others that the faculty of conviction... persistence... mindfulness... concentration... discernment, when developed & pursued, gains a footing in the Deathless, has the Deathless as its goal & consummation; whereas those who have known, seen, penetrated, realized, & attained it by means of discernment would have no doubt or uncertainty that the faculty of conviction... persistence... mindfulness... concentration... discernment, when developed & pursued, gains a footing in the Deathless, has the Deathless as its goal & consummation."

As to why people are stubborn and unwilling to change their views.

That is mostly due to their inate stupidity, clinging to the identity and the relative advantage that comes with their position.

For example

islamic and christian leaders aren't interested in finding truth when they converse, they are concerned with conserving their status in general.

Many people have so much invested interest in their views that obtaining knowledge would be perceived as ruining their good fortune and causing them a lot of distress.

Some religions exert immense social pressure on children to conform to a set of beliefs, even if at the level of lip-service and go as far as calling for execution in cases of apostasy.

As to Buddhists, it is basically the same..

It feels good to think that one has affection & love for The Buddha and is guaranteed at the very least a good rebirth, it feels good to do something thinking it bears a great fruit and it helps one sleep at night.

Well if you're a Christian, then Christianity is always 100% right. If your Muslim, then Islam is always 100% right. It can't be otherwise. Your universe simply doesn't allow the possibility. It's written into the fabric of your existence like acceleration due to gravity in our physical universe. Why wouldn't you be c**ksure if there's no other possible way it could be?

But to be a Buddhist is to stand apart from any universe. If there was no Buddha, no dharma, and no sangha, Buddhism would still be true. If the kalpa ending fire laid waste to everything, it wouldn't matter at all.

From a Buddhist perspective, the Buddha and the dharma only "deals" in Truths.

Buddh[ISM] is a mental construct created by "lesser" people (worldlings) The Buddha himself only explained the truth of suffering, the cause of suffering, cessation of suffering and the path to the cessation. Obviously then "lesser" people created vast religions and even so far as elaborate deity worship around those truths!

To put it simply, these truths are not held by a religion, or by any person, including the Buddha, they are able to be found by anyone at any time. So much is these truths true that they are actually "held" in other religions as well. for example Christianity has ethical conduct and karma (reap what you sow) or sunyata (corinthians 12-13)

So while other religions force these rules or concepts into and onto others, Buddhism only deals in truths, so for damn sure anyone who has seen these truths (sotapanna) can be c**ksure that these truths are true. Other religions (from my own experience) are faith based and conceptual based ideologies... The c**ksureness of those who practice them are often just a form of ignorance as they are unwilling or unable to explore the actual truths of reality.

That is my opinion.

The Buddha discouraged being c**ksure:

MN38:14.1: “Pure and bright as this view is, mendicants, if you cherish it, fancy it, treasure it, and treat it as your own, would you be understanding how the Dhamma is similar to a raft: for crossing over, not for holding on?”
MN38:14.2: “No, sir.”
MN38:14.3: “Pure and bright as this view is, mendicants, if you don’t cherish it, fancy it, treasure it, and treat it as your own, would you be understanding how the Dhamma is similar to a raft: for crossing over, not for holding on?”
MN38:14.4: “Yes, sir.”

Therefore, the Buddha is a counterexample to the OP.

A disciple of the Buddha wouldn't quarrel around but just remember and know DN 1: Brahmajāla Sutta— The All-embracing Net of Views | The Brahmā Net, good householder and use struggles in the world just to increase efforts but Samvega.

I want to address two aspects of this question.

First, why is everyone so sure of his or her own path? I think the answer lies in the process of identification. This is in our nature, identification is even considered necessary to develop a personality when growing up. We can identify with a lot of things. With a person, a religion, a race, a gender, a football team, an occupation, a concept, a political party or system, a role in a family, a role we’re playing in theater or on stage, and so much more.

Identification can happen consciously or unconsciously. When we go to work, we may see ourselves as cook or boss, but completely different when playing with our kids, and again change when sitting down to meditate. An actor has to identify with the role he is playing in order to play it convincingly.

The danger is over-identification. When we identify too much, we may become presumptuous, intolerant against others, even fanatic. I am so convinced of my country, my religion, my political opinion, my brand of car, my race, my football team, my whatever, and my whatever is so obviously superior than your whatever. If you cannot see this, you must be ignorant or brainwashed or outright evil. For the good of all beings, I have to argue with you, convince you, and if I fail, then oppose you, fight you, silence you, kill you.

This is what I observed. I’m no psychologist, so identification may not be the perfect word, but I hope the point is clear.

Second: Doubt in Buddhism.

Having no doubt in Buddhism does not mean to have blind faith. Being a hindrance, having no doubts is an essential part of meditation. Besides doubting the teaching or the religion, which is your question, there will be doubts about other things: Am I able to do it? Do I do it right? Did I get this right? Is now the right time? Did I just have a great achievement? Is my teacher the right one? Why am I doing this? How am I doing?

Doubts are necessary, even helpful, outside meditation. Doubts can arise if my understanding is incomplete. Doubts can and should motivate me to go deeper, to investigate, to look further, wanting to find an answer, a reason, an explanation. And this is a good thing. However, when sitting to meditate, it’s time for the doubt to shut up. If doubts arise, they should be put aside, left for the and of meditation and then reviewed. Then the meaning can be uncovered, and appropriate action taken.

Compare it to a sports team. A coach will teach the team to be absolutely convinced, without a doubt, that they will win - during a match. But outside a match, this could be harmful, as they may not see their weaknesses and thus not work on them.

OP: So to sum up my question, how is it that everybody is c**ksure or their own religion?

The quoted suttas below say that an unenlightened person says certain teachings are true, or holds to views, because of clinging, because he depends on them. Due to clinging, it's hard to let go and see other perspectives. These and other suttas in Snp 4 show how an unenlightened person says his view is true and others are false, calling others fools.

However, the enlightened don't cling to any views and have let go of all views. They see things as they are. They are also free of conceit and have no need to dispute with others and defend their views.

So, does this mean that the Buddha accepted all other religious views in his time and said that they all lead to liberation? No. Definitely not.

As a fully enlightened teacher, he could discern between right view and wrong views. Only the Right View (samma ditthi) can lead to liberation. It's the forerunner of the path, ahead of others, in the Noble Eightfold Path.

There are various suttas where the Buddha was confronted by followers of other teachers or consulted by seekers who spoke to various teachers (like in DN 2), and the Buddha explained why each of their views are wrong, and explained the Right View.

In this answer, we see how the Buddha debunked Jain views on karma in MN 101.

We can see how the Buddha's explanation of the dependent arising of consciousness based on the six sense media and their sense objects in MN 38 easily debunks the explanation of consciousness of Advaita Vedanta as found in the Hindu writings Aparokshanubhuti and Vivekachudamani.

You can find many other wrong views explained in DN 1. Followers of a number of religions today may hold to some of the views in DN 1, especially the wrong view of eternalism.

If a person was stubborn and not interested in listening to the Buddha's explanation, the Buddha simply stopped teaching him (AN 4.111).

As stated clearly in Dhammapada 254 - 255, noble (ariya) contemplatives cannot be found outside what was taught by the Buddhas. In other words, you can only find noble contemplatives where there are the four noble truths, the noble eightfold path, the three marks of existence and dependent origination. Only these lead to the permanent end of suffering.

Also: "But in whatsoever Dhamma and Discipline there is found the Noble Eightfold Path, there is found a true ascetic of the first, second, third, and fourth degrees of saintliness." (DN 16)

In fact, the specific person of Gautama Buddha is not more important than his teachings, which was discovered and rediscovered again by multiple Buddhas (SN 6.2) in history.

Even if Buddhas never arose, the three marks of existence is in any case a natural law independent of the person discovering and teaching it (AN 3.136).

The term noble (ariya) by the way is a technical term that refers to people, teachings and practices that lead to Nirvana, as explained by the Buddha. Please also see "What is ariya or noble?"

A person entrenched in his teachings,
honoring a preconceived view,
isn't easy to discipline.
Whatever he depends on
he describes it as lovely,
says that it's purity,
that there he saw truth.

The brahman, evaluating,
isn't involved with conjurings,
doesn't follow views,
isn't tied even to knowledge.
And on knowing
whatever's conventional, commonplace,
he remains equanimous:
'That's what others hold onto.'
Snp 4.13

Because entrenchments in views
aren't easily overcome
when considering what's grasped
among doctrines,
that's why
a person embraces or rejects a doctrine —
in light of these very
entrenchments.

Now, one who is cleansed
has no preconceived view
about states of becoming or not-
anywhere in the world.
Having abandoned conceit & illusion,
by what means would he go?
He isn't involved.
Snp 4.3

Sine you've asked a not-exactly-Buddhist question, I'm going to begin with a daoist observation and work my way back to Buddhism. Chapter 38 of the daodejing contains the following passage (variably translated):

When the Dao is lost, people become good. 
When goodness is lost, people become moral. 
When morality is lost, people turn to ritual. 
Ritual is the empty husk of true understanding;
After that all is chaos.

My point is that on the spiritual path we must take this declining list in reverse.

• That chaos is nihilism, where nothing has meaning or value beyond the immediacy of ones own desires
• The step up from nihilism is transactionalism, where we do what we think we need to do to get what we want from the world
• Up from transactionalism is conviction, where we have strong, rigid, deeply held beliefs about how the world is and should be
• Beyond conviction we find compassion, where all our beliefs are tempered and softened by our knowledge of human suffering
• And past compassion there is understanding (attainment, right-view)

Every faith (at its philosophical core) teaches that upward path, but in practice religions and religious practices can get caught at any of those lower stages. Spiritual leaders can sometimes become so attached to elevating themselves and their followers that they push themselves downwards towards brutal dogmatism. But in any case, being 'c**ksure' of one's faith is a natural and perhaps unavoidable stage in spiritual maturation, one that will (hopefully, eventually) be set aside for a better understanding.

How is it that everyone's c**ksure of their own religion or spiritual path?

I'm not sure they are.

One of the mottos from the Christian canon is, as a prayer, "I believe; help my unbelief!" (Mark 9:24) -- which I read as an admission of not being c**ksure.

Perhaps Buddhists too aren't "c**ksure", instead I think the canon keeps tells people to assess the Dhamma carefully and not to take doctrine only "on faith". There are many elements of doctrine, and I don't know about you but I tend to take the Noble Truths as core, conversely not pay much attention to some other elements like Mount Meru for example.

So I think it's not true, or an exaggeration, to say that all Buddhists are c**ksure about the same entire ontology, and all Christians are likewise about a different ontology. Instead each person has different views which they're more or less faithful to.

You could instead ask, "Why are people sure about the things they're sure about?"

• Firstly, that's just a truism -- i.e. people are "sure about what they're sure about" by definition.
• Second, this latter phrasing of the question removes some of the antagonism from the original questions (i.e. people contradicting each other).
• As for "why?" I imagine it's because they find the belief helpful in some way -- possibly reassuring, or some other kind of "good".

most of us who do sincere practise are totally convinced with the Buddhist ontology and overall framework

So I think that people find or build or select an ontology which they find convincing, except that instead of "convincing" I'd say "workable", "reasonable", "useful", perhaps "enlightening" or "liberating".

we were ready to die and kill each other for our religious beliefs

That might be an exaggeration too. Maybe people killed each other for other reasons -- political and economic -- and "religion" was a convenient excuse but superficial, just one of the "identifying" characteristics that sometimes happened to distinguish "us" from "them".