Why do Christians presuppose the Bible is their God's word?

Because we believe it. 40 authors over 1500 years with a central theme? What is it about belief that confuses you?

Who else's would it be?

It's a belief that exists primarily via generational childhood indoctrination. There's no actual logic or argument for it. And plenty against it, as you point out.

Because Jesus Christ clearly demonstrated that He was God, by constantly doing things only God could do. He, God, founded one Church, said it was to remain one, and promised that one Church "The Holy Spirit will guide you into all truth, and "Whatsoever you bind upon Earth is bound in Heaven", and "He who hears you hears Me". That one Church, with its divine guarantee of truth, compile the Bible after studying and discerning hundreds of early writings for six years, and declares as binding truth that these 73 texts are the inspired Word of God. Therefore, God Himself has declared it to be so.

In the Dark Ages, one would be killed (in the most horrible way) for suggesting that the bible was not written by God.  This is why the religion is still going strong.

Brainwashing, and weekly indoctrination for life. 

Gov't is killing you with highfrequencyelectricity. Theotokos Virgin Mary gave prayers to "Schema-nun Antonia" on how to save aborted babies from hell. If you pray these prayers diligently, aborted babies are released from hell. On each painted nail there are forty demons. Smoking is censer to the devil.Using foul language calls upon Pagan deities (aka demons); Holy Spirit departs on seven meters. People who use drugs see demons who cleverly disguise themselves as ghosts and aliens.America will be last country to switch to Euro (antichrist's world currency). Contraceptives = abortion;using contraceptives for one year = five aborted kids.Miscarriages happen because of high heels; cesarean because of pants (second generation cesarean will be infertile).Unbaptized aborted/miscarried/unborn babies burn in hell until Final Judgement; if pregnant, keep the kid and deliver at home because kids are chipped using IVs and vaccines in hospitals. Dentists and doctors chip patients secretly. Ultrasound leads to mark of the beast; don't do ultrasound, please. Abortion leads to breast cancer; a demon is released from hell for each aborted kid. Dogs can become possessed; don't keep dogs inside your home [Pelageya of Ryazan]. Walmart has technology to administer mark of the beast to those who have cat bacteria in their stomachs/brains; stay away from cats [Athanasius3rdofConstantinople]. Next false flag is the Statue of Liberty. Above earth there is ice (hemisphere); when rockets go up they bring ice down from upper sky to lower sky; ice stuck in lower sky will fall on us during Apocalypse. Earth is flat; earth stands on three stalactite pillars (The Most Holy Trinity); pillars stand on water at zero Kelvin; underneath this ice there is a bubble; and then the abyss. Zodiac is planetary prison of demons; don't believe in horoscopes or you'll exhibit the traits of the trapped demons. Most thoughts and dreams are from demons; demons never do good. Sleep fully clothed; pray the Jesus prayer. Pray to your guardian angel to have normal sleep. Vyacheslav Krasheninnikov was the last prophet before Apostle John (who wrote the Book of Revelation), Enoch, Elijah, resurrected Seraphim of Sarov, and resurrected Sergius of Radonezh will preach against the antichrist. Humans were created about 7530 years ago. Birds participate in time creation. It's a sin to kill birds. Dinosaurs live under our level; they will get out through sinkholes and lakes; to kill them, go for their nerves. Save the birds; but kill the dinosaurs. First dinosaur will come out of Volga River in Russia. Scientists don't see dinosaurs under our level because of radiation. Sinkholes happen because people dig for resources underground and because earth is heating up. Demons grow human skin (from a sample taken during abduction) and put it on so as to look like us. Demons will invite people to be healed inside their UFOs; those who go will be like zombies after. Gov't provides demons with diamonds and allows demons to abduct people. If you're being abducted, slowly pray the Jesus prayer. Don't panic. Demons use diamonds and souls to power their UFO craft. The bigger the diamond, the more it lasts. Demons have 4 UFO bases: 1)moon 2)inside fake mountain Kailash (Tibet) 3)in lake Baikal in Russia 4)in Atlantis which is underneath double ocean floor of Mariana Trench (Pacific Ocean). There are no aliens. Nobody lives on other planets. Airplanes that go down are hit by demons because they need the airspace to fight Jesus. Antichrist is pale with red eyes. He's possessed by Satan since he's 12 years old [Lavrentiy Chernigovskiy]. He flies super fast; deceived people will say: "Christ is here; Christ is there" when he's flying from one city to the next very fast. He wears gloves to hide long nails. He'ssurrounded by demons who appear as angels of light. Antichrist will trick people that he can do mountain moving and resurrection using holograms/hypnosis; fire from the sky is real because of pollution gases in the atmosphere. Antichrist will have food only for 6 months; then he will feed his 666ed people flourfrom mashed up turtles (Tavrion Batozskiy), but this won't be enough because 666ed people are 10 times as angry and 7 times as hungry as normal people even though 666ed people became shorter (3-4 feet tall = 80 - 120 cm)because nanochips do function of organs (organs diminish) [Nilus Myrrhgusher]. If you have a lot of nanochips in your forearm, then you will not be able to make proper Orthodox sign ofthe cross (last mercy for you will be to cut yourforearm off). Nanochips are sprayed by the gov't usingchemtrails; they're also in gov't food andmedicine; so, eat food from your own garden. In case garden is destroyed by ice from the sky, have chickens for eggs and goats for milk (Paisios). Lipstick contains cells of abortedfetuses, dog fat, and placenta; human fleshis in McDonalds, Pepsi, toothpaste, antiaging,anticancer, vaccines, perfume, etc.; that's why you should not be using anything that modern society has to offer. You're better off hiding within a 10-15 people group in order to escape Apocalypse. During Apocalypse,Christians will eat dirt from under pussywillowtree as it's filled with tears of Theotokos VirginMary; this water will flee if a 666ed person tries to get it. Barcode is Druid black magic curse; QRcode is Mayan curse; when food is scanned, it becomes dead because laser is a substance from demons. Don't go into a UFO to be healed by demons. Unforgivablegreen666 is given by isotope rays on wrist or forehead when people stretch hands to receive small plastic grey card with no name on it (WorldPassport). It doesn't just have to be during this procedure (could be anything you sign up for or anywhere where there is a secret scanner); biometrics (fingerprints, eyesscan) or getting picture for passport are very dangerous because they could mark you secretly. GabrielUrgebadze said thattheydoitonindexfingerwhenthey scan your finger. Basically, trytoavoidnewdocuments at all cost. Policewill microchip and isotope ray people on highways. Chipped people will be influenced by computers to take grey plastic card; but when they do, greenmark by isotope rays is given on forehead/wrist. Food stores will isotoperaypeopletoo. Antichrist will also release prisoners/insaneasylumpeople tomarkpeople. Reject 666 at all cost because it leads to permanent hell. If you're about to be marked, pray the Jesus prayer. Hide with OrthodoxChristians to escape 666; leave all electronics behind so that antichrist's minions can'ttrack you; burn documents because they're fromSatan. TheMostHolyTrinity gives you a name during baptism; devil gives an antiname during antibaptism (ex. SocialSecurityNumber). People who die with these Satanicdocuments go to concentrationcamp in hell to await FinalJudgement; once the BEASTComputer is burned down, souls will be released for FinalJudgement. That's why you should give back documents of your deceasedrelatives back to the gov't so that the gov't cancels these digital antichristiannames given during antibaptism by the beastsystem; or just burn thesedocumentsbecause gov't could get upset and could send demons to mark you because of this outright act of defiance. Prophecy from halfamillennium ago describes FinalJudgement like this: Jesus was very upset with people who had littleboards (plasticcards) in their hands because they wanted discount from the antichrist. Givetocharity in the name of ArchangelMichael; he rescues people from temporaryhell twice a year [at midnight September18-19 and similarly on November20-21; pray at these times on your knees remembering the deceased by names (adding "and relatives by flesh up to Adam") so that they arerescued if they're in hell] (or brings them up alevel, that is, to a level with lesspunishment;eventually, people arefreed). Feed thepigeons;when pigeons bowdown, people are savedfrom temporaryhell. It's a big sin to rememberthe deadwithwreaths (because demons put these wreaths on their necks if they're inhellwith theirhands tied up behind their back while hanging by their wrists), meat,alcohol,sweets,andworldlymusic. Demons print icons of saints/crosses/8sidedstarofTheotokosVirginMary in newspapers/products so that you throw these newspapers/products in the trashblasphemingthesesaints. Crosses on solesofshoes and backofpants are blasphemy. Demons makecarpets with crosses and put them on sidewalks so that people walk on crosses. Playingcards mockhow Jesus sufferedonthecross: clubs(cross on which Jesus wascrucified), diamonds (fournails Jesus was crucified with), hearts (sponge with vinegar that Jesuswas given to drink),spades (spear with whichJesus waspierced).Cremationisdevilworship; only blasphemers such as Lenin should be burned; if Lenin is buried, earth will be polluted, and China will attack Russia because of this. AfterChinaattacksRussia, RuskiOrthodoxTsar (shown by resurrected SeraphimofSarov) willcome topowerinRussia;this Tsar will slay traitors inside church and gov't; asaresult, Russia will be the only country not under the antichrist.Ecumenism=263heresies;each heresy leads to hell. In 2006 in Moscow (that's why Moscow will sink),representatives from most religions signed a document where it says that all religions worship the same SupremeBeing [aka thedevil]. Priests who participate in ecumenism will have Pagans walking on theirheads in hell. ArkhimandriteAntoninKapustin left a prophecy that Church of All Russian Saints in Gorny Monastery, EinKarem, Israel will be blessed by forerunner of antichrist; pseudo-PatriarchKirillofRussia and MarkofBerlin blessed this church in 2007. When priests pray for current gov't (instead of praying for future Tsar),Jesus gets up from His throne and turns His back to them. Forgive me

Read here: https://www.jw.org/finder?wtlocale=E&docid=2010160&srcid=share

It always strikes me how Christians can't understand that the bible is wholly uncompelling to someone who doesn't believe in it (and would be for them as well, had most not been taught from a young age to revere it).

😁 It is hard to understand that God knows us so well that he know what happens to us ten years before it happens! 😇
And also God talks to us in and relays messages to us in our own language in the way we talk because he knows us better then we know our selves. So you would think it was that other person talking to himself and writing what he says! 🥳
When it was God talking all along! 😭