Yes. Christianity is testable.
I'm writing the following as a committed Christian. It is based on the core [fact | assumption] (choose one as desired) that the God of Christianity exists. Many will not agree with the truth of that assumption. For this to be useful to you, assume it for the purposes of the argument for now, and read on.
So, yes, Christianity is testable. However, you cannot "put God in a test-tube", arrange hoops for Him to jump through, demand that he queue for examination or use the Scientific method to conduct enquiry, except on His terms.
This is not the 'cop out' that Brondahl reasonably identified with his 'Ah, yes, the classic "it's testable ... but only if you believe. And if you don't believe the He will choose not to satisfy your tests.' - but it's a fair observation. Doing it God's way is required - and why should that be a surprise.
The Bible notes in a number of places that "miracles" can be expected.
Mark 16:20 claims specific occurrence as part of an evangelism 'campaign' "with signs following ...".
*From here
"In the Chronicles of Narnia, when confronted by the idea of Aslan, the lion, who is a picture of God, Lucy asks, "Is He safe?"
"Safe?" said Mr. Beaver."Who said anything about safe? 'Course he isn't safe. But he's good. He's the King, I tell you."
Mr. Tumnus also says, "He's wild, you know. Not a tame lion."
What Lewis is saying with metaphor is that God does what He wants to do for His purposes.
And sometimes that may indeed include demonstrations for whatever purpose that demonstrate testability. Or to meet specific requests or needs.
Understandably, that will seem a very evasive and weasel-worded claim.
So.
I'm a professional engineer. I'm 72 years old and still professionally active.
I have a Master's degree in electrical engineering, life long experience in many areas that my professional core capabilities have taken me into.
I live in NZ. I've carried out development and production in China and Taiwan - around 15 business visits to Asia and many a fun hour spent trying to ensure that people build things the way I've designed them.
I understand numbers and statistics and probability "well enough" - In a prior corporate lifetime I carried out numerical and operations analysis for a diviion of our country's then largest corporate. I value integrity and honesty.
And: On a relatively small number of occasions when it was a really really good idea and available options were non-existent, I've gone to God with specific well defined requests, in some cases written down to ensure that I know what I'm asking for. I never assume that I'm going to get what I'm asking for. And/but, when it's serious or useful or interesting enough to be done, I have had outcomes that are statistically impossible by any usual standards. Many many many standard deviations off the norm. Probabiities that make me laugh (sometimes literally). On the few occasions when Ive shared this I have had, very understandably and very reasonably, people suggest that I am cherry picking, identifying a few good outcomes from many trials, and all the other statistical "tricks" which people accidentally or purposefully fall into.
All I can say is - "No, I'm not". I'm happy that the laugh out loud small volume specifically asked for 'impossible' outcomes do meet an 'impossible' standard.
[Added - 231201] : I haven't kept count, but I'd say it was in all or most cases of this very special approach. A response other than hoped for is of course always possible, but even those could be couched in statistically "impossible" ways.]
But, wait, it gets worse.
For reasons which I will explain, and which are genuine, and which people will understandably be unhappy with, I'm not going to provide any details for others to test. The best I can offer is that others 'come and try it'.
This will usually require a journey in life to get to the "with signs following" stage, but how long each such journey is is God's choice. For some it may be on day one. Or before.
Why not describe at least some of my experiences?
Largely it's because my understanding is as C S Lewis describes.
God is not a "tame Lion". IF he wished to arrange for street corner miracles at 5pm on Fridays then he would. He doesn't. Usually.
To add context, I "have God in mind" to a variable extent a lot of the time. Not usually 'praying' as most would identify the word, but aware of him in day to day life. I may share thoughts on needs or outcomes in the world which seem desirable from my lowly perspective. This is much different from the sort of situations I described above. This is over time "high volume' and I don't "keep score" or have strong specific hopes or expectations. I say the above to distinguish it from the "testable" area above. I've not see an amputee given a new limb. I'm well aware of the comments made in this sort of area. So far my "testable" requests have only covered the impossible - miracles may yet join the list :-).
Added:
JMac commented:
"Doing it God's way is required - and why should that be a surprise." I dont find it surprising that such a claim is made. But quite frankly, I dont see any value in those types of assertions. Anyone can make such claims about any number of unreasonable beliefs to explain why they cant be tested. ... To me, such claims are the opposite of convincing and make me more suspicious of the claims.
Response: While this answer one specific query it also usefully addresses the reasonable 'objection' wich is very frequently raised:
@JMac I sympathise with your position and agree that it "would be nice" if it was more black and white. I can also see, and I assume that you can too that IF 'the christian God exists' (given for the purpose of the argument) then IF God does it this way then there is 'good reason' and that the choices are to either 'follow the program' or to ignore it.
However, I have offered slightly more than the above - albeit not enough to meet the usual objections for most. I CLAIM to be a person who values honesty and integrity. I could provide "character references" but that's not usually really going to help. I CLAIM to be an experienced engineer (that's provable) still active (same) & I CLAIM I'm experienced in numbers, statistics & probability and that I held a job in a large corporate using those abilities. Also provable.
THEN I CLAIM that I have a relatively small number of cases where I've asked God for specific (sometimes documented) things and received statistically and probabalistically "impossible" results. I then "spoil it" by saying I'm not going to give any details. Most will at this stage quite understandably give up, and dismiss me as (yet another) loony/liar. But, some will believe me enough to follow through.
That's why I bothered writing this answer.