Is it possible to be both a humanist and a Christian?

Just as a note (and I, personally, find it the most problematic of the humanist position), but this quote cannot, in any way, be reconciled with Christian thought:

To be ethical, acting in a way that puts human welfare at the centre of morality

As a Christian, human welfare cannot be at the center of your morality. The first and greatest commandment, the ground of the Christian's morality, is, instead, the love of God.

To answer the question directly:

The important point in the 4th item is "this one life": meaning - we shouldn't be looking to some reward/punishment in some promised afterlife (or: staus in reincarnation, for some religions) to justify or rationalise our actions in this life. Rather, we should do things because they are reasonable/ethical things to do, and because we enjoy them in this life. Essentially "No, Mr. Pascal, we do not agree with your conclusion to your wager/gambit". A brilliant example of this is the archbishop of Canterbury, 1965:

a nuclear war would involve nothing more than the transition of many millions of people into the love of God, only a few years before they were going to find it anyway

To the Humanist this is abhorrent. The destruction of everything is not to be celebrated, and no effort should be spared in denying such a horrific end from occurring. Sadly there genuinely are people (of all religions) who actively seek to bring about their own traditional "end of days" scenario. I've seen enough people, even here, excited by "won't it be great" and "isn't it good that it will be soon" to be very concerned about that eagerness spilling over into... manufacted world death.

The third is tricky - this may also include those who practice things that your beliefs don't agree with. Now, many Christians are great with this. But sadly many others aren't (I could point to things like Westboro Baptist Church). I don't believe for one moment that this is a single brush to tar all with: but - it is real and exists all too commonly.

(rephrased) Another issue with human value is the slightly thorny issue of "total depravity", which might be expressed that people are universally wicked, and that only by following the stated path (which path. exactly varies between sections) can people earn redemption. Whether this is faith/acts/both/etc I won't ponder; however, this conflicts with the Humanist position of emphasing first the value and worth of all. The positive, rather than the negative.

Flimzy notes the flaw in the second; putting God at the centre vs putting humanity at the centre.

The first comes down to how you interpret "rational". In the context used, it is meaning "non-supernatural". This isn't me saying "all YEC are idiots!" (which I have been accused of implying before): it is simply saying that - given something we don't yet know, assuming a supernatural origin is not the rational (meaning: based on scientific-method, deductive logic, etc) / natural thing to do. You are, of course, welcome to do so, and many have looked at ways to satisfy themselves that this is reasonable. I would argue that in the context used, supernatural beliefs (without evidence etc) are the opposite of the way that line means "rational". I think this is neatly summarised by I tweet I once saw:

Dear Religion, Pics or it didn't happen. Love, Science.

Actually, a big point here is the "h". If you mean small-h humanism, then yes: there are various Christian humanist movements. If you mean, as I was referring to, big-H Humanism, then that is referring to secular Humanism, which explicitly does not give any special meaning to religious beliefs. In particular, it (wiki) applies:

specifically rejecting supernatural and religious dogma as a basis of morality and decision-making.

I honestly haven't ever considered whether one can be Humanist and Christian at the same time; I don't really think so; Humanism respects the believer and fully defends the right to believe - but doesn't give credence to the supernatural beliefs themselves. Which is typically exactly how Christianity feels about virtually every other belief, so we're not in hugely different places there (I say "virtually", because it gets a bit wooly re Islam and Judaesm and the OT).

Perhaps the bigger issue is: how do you look to find explanations for the unknown? If you look the the Bible (creation, Noah, Babel, resurrected messiah, etc) then that is fundamentally at odds with Humansim, which looks strictly to ... er... (picks words carefully) "research that looks only to strongly verifiable evidence" rather than "tenacious tales, of uncertain, unverifiable, faith-based, belief".

That said: one could well be a Christian who is pro-secular in terms of public policy (and in fact, a MORI survey in the UK indeed showed that the majority of UK citizens who identified as Christian feel this way - of course, it also showed that the majority of the same set of self-identified Christians were actually non-religious cultural Christians).

Re compatibility: if your decision making doesn't make (emph) any (/emph) consideration of your religious beliefs, then I would propose you aren't actively applying Christianity in your daily life. In many ways Christianity is pretty easy-going in terms of daily life (contrast the strictness of Judaism or Islam, for example) - so this distinction is quite subtle.

I should also emphasise that that list is not "mine" - it is, as I believe I mentioned at the time. I'm sure the actual list is searchable - IIRC it was a response to a Christian membership-card. What I think the list does show is that Humanism does focus on ethics, fairness/equality, and the value of the person. This is not unreasonable.

Perhaps the acid test is simple (the following is mine - quoted only for emphasis):

Your chosen supernatural-inspired religious text states (without reasons), categorically, clearly and without room for confusion, that activity X is Wrong with a big W. do you:

1. accept this, and actively seek out to undo all X
2. accept this, and not participate in X
3. accept this, but (if you happen to like X) do X anyway, knowing that it conflicts, but that "in balance" you're OK; maybe feel a bit guilty and embarrassed, secretive, maybe depressed, as a result; alternatively and equally - deny yourself X and feel repressed
4. ignore this, and participate, or not (as you choose) in X, since the text is of no importance
5. ignoring the supernatural aspect, look to understand X, and where this may have come from; is there an underlying reason that avoiding X benefits society? Or perhaps benefited society at that time? Is this a hangover of an early culture, with the culture biases of the time? Is there a valid and fair reason to deny X now (again, supernatural aside)? If we assume human origin, was there perhaps political/personal gain behind this law? (Important:) If there is no logical argument to deny X, then actively seek to challenge those that enforce X on purely supernatural grounds, even when X has (by history) been absorbed into law (human law can, after all, be changed - it takes time, though)

The first three would seem to be compatible with Christianity (orthodox to liberal, in order); the fourth seems rather hedonistic on the surface; the fifth is the Humanist position. It certainly doesn't use "God" as a definition of morality (or, if you prefer, "ethics" - although it is primarily religion that distinguishes the two terms).