Coulda, woulda, Buddha

"Death with Life contended: combat strangely ended! Life’s own champion, slain, yet lives to reign".

It's important to first note that this above quote is taken from a roughly 500 year old (if not older) Easter sequence regarding the crucifixion of Christ for the sins of his people called the Victimae Paschali Laudes. It is referring to Christ's death on the cross and points to his resurrection.

Buddhism teaches the exact same concept

Unless you're somehow suggesting that Christ's death and resurrection is central to Buddhism, you're far astray here.

And if you are suggesting this, or something similar, then what distinguishes Buddhism from Christianity in your mind?

good action in life means that death is not the struggle we believe it to be, that beyond death there is more.

Nowhere in the quote you cite is there a suggestion that living a good life implies less struggle in death. Let alone that there is anything more beyond death. Rather this is good evidence of your reading into the quote rather than allowing the quote to speak for itself.

Even the concept of hell is similar really, in Buddhism you go around in circles if you do not improve yourself, you are destined to come back again and again until you realise your existence and learn mindfulness.

Except that this is contrary to traditional Buddhism as well as traditional Christianity. For one, according to Christianity hell is interminable. But according to your own reapplication of the term to fit Buddhism, hell is going around in circles until you get it right, so to speak. In Christianity, there are no second chances whereas, in your reapplication of the term to fit Buddhism, there are. This is just the most obvious point.

If you attain that, then you achieve enlightenment forever, and do not return.

Common to all Buddhists is the belief that suffering is caused by desire, and enlightenment is the extinction of desire. According to Christianity, however, heaven is not the same as enlightenment. Heaven is not the extinction of desire, either in this or the next life.

And Christianity has no problem with certain desires and passions in this life.

What's more, Christians are not saved by self-enlightenment or some other gnostic means. Christians do not enter heaven by any merit or by anything at all in or of themselves, but solely by the grace of God in Christ Jesus.

Nor is heaven some sort of transcendental state of bliss. Although it's true being in heaven will make one joyous, heaven itself is not equivalent to joy and bliss. Heaven includes joy and bliss, but it is not defined as joy and bliss.

Lack of good practice in Christianity means you are denied entry to 'eternal life' - which is the same thing in my mind.

Actually, sin is what causes someone to end up in hell, separated from God, and in eternal punishment and torment. That would include but is not limited to "lack of good practice."

Does this mean I believe in God, rebirth, and enlightenment? Should I counter that there is no proof of any of the above?

How do you define terms such as "God" and "proof"?

Should I point out that there was no proof of gravity a thousand years ago, but that we could feel it anyway?

By chance are you suggesting that you're an empiricist?

What I do know is that people actions over thousands of years have continuously been greater than themselves, that we can learn so many good lessons from them.

Sure, we can learn good lessons from people. But we can also learn bad lessons from people.

Why do you suppose people's accumulated actions over a long period of time are (morally?) greater (or worse) than the people themselves? Especially when they've resulted in things such as pogroms, the Holocaust, Killing Fields, and the like?

And really, God can be anything you like. It can be the universe, it can be the sum of mans actions since their existence, it can be a chocolate pudding that makes you happy,

If God can be anything I like, then can God be me? I am God! Is that what you're saying? If so, what a self-centered way of looking at God, wouldn't you agree?

But your definition of God is meaningless at best. If I can define God however I like, then what's the use of definitions for God? And if there's no use using definitions for God, then why listen to what you have to say about God in the first place?

hell, it can mean non-existence if you like.

Or, if I like, can it mean separation from God and all that is good? Or how about eternal punishment in a burning lake of fire? Or how about a deathless death, forever and ever, due to your sins? Or how about the place where the souls of the wicked are cast, and remain in tormet and utter darkness, reserved for judgment on the day of judgment (per the WCF and LBCF)?

No church in my mind has defined God in any real terms,

For starters, I'd suggest attending an actual church rather than having church in your mind. Who knows? You may begin to learn a thing or two about God.

In the meantime, this definition from the LBCF should more than suffice:

The Lord our God is but one only living and true God; whose subsistence is in and of himself, infinite in being and perfection; whose essence cannot be comprehended by any but himself; a most pure spirit, invisible, without body, parts, or passions, who only hath immortality, dwelling in the light which no man can approach unto; who is immutable, immense, eternal, incomprehensible, almighty, every way infinite, most holy, most wise, most free, most absolute; working all things according to the counsel of his own immutable and most righteous will for his own glory; most loving, gracious, merciful, long-suffering, abundant in goodness and truth, forgiving iniquity, transgression, and sin; the rewarder of them that diligently seek him, and withal most just and terrible in his judgments, hating all sin, and who will by no means clear the guilty.

Continuing:

other than he's referred to as a guy, and depending on you speak to, is either kind or loving, or totally vengeful. It's up to us to decide I suppose.

Let's assume there is a God. If there is a God, then is it really "up to us to decide" who or what he is? Wouldn't it rather be up to God to decide who or what God is?

Otherwise, if God does not exist, then does it really matter what definitions we might concoct for ourselves, however appealing to us these definitions might be?

"God" makes people do much good, and so much bad. And if all of this is in the name of God, then the meaning of that word truly does belong to us.

Does "God" make people do "much good" and "much bad"? Is that a fact? If the Son of Sam murders a victim, is it the Son of Sam that murders the victim or is it God making the Son of Sam murder a victim? If you help a little old lady across the street, are you helping a little old lady across the street or is it God making you help a little old lady across the street?

Also, for example, are things such as persuasion or influence the same as, say, compulsion?

So am I Catholic? Buddhist? Both? (You can be both).

How so? Please spell it out for us.

And according to whom? According to you? Or according to, say, the Pope or the Magisterium?

Do I need to define myself, if even God can't be?

At this point, it'd be a pleasant surprise if you could define anything at all.

These definitions of our religious leanings only exist to make other people feel comfortable (or uncomfortable). I'm happy with no definition, although I get really pissed off when people call me atheist or agnostic because I'm neither, those words were made up by people to create definable religions where there were none.

Definitions might also exist so that we can communicate with one another. And to distinguish truth from falsehood.

Then again, maybe you're right, why bother with passé things like definitions, communication, and truth and falsehood if it makes people uncomfortable?