So basically the Gnostic writings are invalid because they run counter to the canon?
In considering the Gnostic writings invalid, Biblical Christians do not focus so much on the idea of "the canon" in itself as on why the writings in the canon were included in the canon, and why the Gnostic writings were excluded. The different writings in the New Testament were all written for the early church, which knew and trusted the writers (and so kept their writings) because the writers were eyewitnesses of Jesus Christ (2 Peter 1:16, 1 John 1:1-4, 1 Corinthians 9:1, John 19:35, John 21:24, Luke 24:48, Revelation 1:17-19) or their immediate followers (Luke 1:1-2, Hebrews 2:3). The Gnostic wiritings, on the other hand, contradicted the testimony of the eyewitnesses of Jesus Christ with regard to what he taught. For the Gnostic writings reviled YHWH as an evil god whereas the eyewitnesses of Jesus Christ showed that Jesus Christ confirmed that YHWH is the one true God (Mark 12:29-31, quoting from Deuteronomy 6:4-5 and Leviticus 19:18). Also, the Gnostic writings denied that Christ is in the flesh (2 John 1:7), whereas the eyewitnesses of Jesus Christ showed that Jesus Christ affirmed that he is in the flesh (Luke 24:39, John 1:14, Romans 1:3, Hebrews 2:17). Also, the Gnostic writings claimed that the Old Testament was to be rejected because it was written by followers of YHWH, whereas the eyewitnesses of Jesus Christ showed that Jesus Christ confirmed that all of the Old Testament is true (Matthew 5:17-18, Luke 24:44-48). Also, even where the Gnostic writings teach something not in the Bible which does not contradict what the eyewitnesses of Jesus Christ showed that Jesus Christ taught, the Gnostic writings still do not pass the test of Biblical Christians being able to recognize if something which is taught is from Jesus Christ (John 10:4,27) or from "a stranger" (John 10:5). Biblical Christians are able to do this because they have received God's own Holy Spirit (1 Corinthians 2:11-13), Jesus Christ's own mind (1 Corinthians 2:16).
So God picks who gets to be a Christian and who doesn't and at the same time who gets to understand the Bible?
Yes. The wholly-corrupt nature of all humans in their natural state (Romans 3:10-12) makes it impossible for any human to ever become a Biblical Christian through his own will (Romans 9:16, John 1:13, 6:44) or his own intellect (1 Corinthians 1:18-2:16). The natural minds of humans can only see the things of God as foolishness (1 Corinthians 2:14, 1 Corinthians 1:18). Biblical Christian faith can come only by God's grace as a miraculous gift from God (Ephesians 2:8, John 6:65, 1 Corinthians 3:5), and he gives this gift only to his elect (Acts 13:48), who are those individuals who were chosen (elected) by him before the foundation of the world to become saved by him at some point during their lifetime (Ephesians 1:4-11).
I guess by extension you are trusting the early churches judgment on the credibility of the early writers but if Christians can really tell the difference then that wouldn't be an issue.
Biblical Christians can trust the early church's judgment on the credibility of the writers of the New Testament for the reasons which have been given: the early church knew those writers to be eyewitnesses of Jesus Christ (2 Peter 1:16, 1 John 1:1-4, 1 Corinthians 9:1, John 19:35, John 21:24, Luke 24:48, Revelation 1:17-19) or their immediate followers (Luke 1:1-2, Hebrews 2:3). Also, the early church had received God's own Holy Spirit (1 Corinthians 2:11-13), Jesus Christ's own mind (1 Corinthians 2:16), so it was able to know whether a teaching of the writers was truly from Jesus Christ or not (John 10:27,4,5), just as Biblical Christians can still know this today for the same reason. Also, Biblical Christians today, just as the early church did, can confirm for themselves that the writers of the New Testament agree with what the Old Testament taught and prophesied (Acts 17:11, Acts 26:22-23, Luke 24:44-48).
Everything up to here kinda makes sense but how would an early Christian tell the difference without the Bible to guide them? How would they not fall into the trap of only keeping the parts that they agreed with already?
Early Biblical Christians had the Old Testament to guide them. As has been said, they were able to check every teaching claiming to be from Jesus Christ against what the Old Testament had taught and prophesied (Acts 17:11, Acts 26:22-23, Luke 24:44-48, 2 Timothy 3:15-16). Also, even if a teaching didn't contradict the Old Testament, because early Biblical Christians received God's own Holy Spirit (1 Corinthians 2:11-13), Jesus Christ's own mind (1 Corinthians 2:16), they were able to know whether such a teaching was truly from Jesus Christ (John 10:27,4) or only from some "stranger" (John 10:5).
A quick example would be the Bible claiming that PI is equal to 3 or that the world is a circle (rather then a sphere). These things by themselves are not a big deal given the time period in which they were written but it does lend doubt to the idea of divine inspiration.
The divinely-inspired Bible (2 Timothy 3:16) doesn't claim that pi is equal to 3. All 1 Kings 7:23 says is that a brass basin 10 cubits in diameter had a circumference of 30 cubits, which could simply have been rounded down from 31.4 cubits. Or the 10 cubits could have been rounded up from 9.55 cubits. Or neither the 10-cubit figure nor the 30-cubit figure was meant to be taken as an exact figure accurate to the hundredth of a cubit. For 1 Kings 7:23 is not giving an exact blueprint description of the basin, nor is it giving a geometry lesson; it is simply giving a general account of how big the basin was. Also, the Bible doesn't claim that the earth is a circle instead of a sphere. Isaiah 40:22 is simply referring poetically to the circle of the horizon as it appears from the surface of the earth.
Note that these are only two examples out of many and there are also the contradictions to consider.
What has been proven to be a false fact or a contradiction in the Bible?
Would the snippets of the Bible count as something that sounds better?
Snippets of the Bible wouldn't count as "something that sounds better" in the sense that that phrase was intended, insofar as when it was said that "one of the prime aims of the devil is to get humans to deny God's word the Bible and start believing something else which sounds better to humans (Genesis 3, Matthew 16:21-23)", by "to deny God's word the Bible" was meant to say that the Bible isn't true, and by "start believing something else" was meant to start believing something else besides the Bible which contradicts the Bible.
That's what I've seen Christians do, only look at the parts of the Bible that agree with their own view.
Can you give an example of Christians only looking at the parts of the Bible that agree with their own view? Biblical Christians look at every part of the Bible because all of its teachings were given by the inspiration of God, so that all of it is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be perfect, thoroughly equipped to perform all good works (2 Timothy 3:16-17). It is only the Bible which is able to make humans wise unto salvation through faith which is in Jesus Christ (2 Timothy 3:15, Romans 10:17, Acts 13:48, James 1:18, 1 Peter 1:23-25).
And you trust the Bible because the early Church trusted the Bible?
Because the early church trusted the Bible is one of the reasons that Biblical Christians trust the Bible. Another reason which has been given is that Biblical Christians have received God's own Holy Spirit (1 Corinthians 2:11-13), Jesus Christ's own mind (1 Corinthians 2:16), and so are able to know that the teachings in the Bible are truly from God (John 10:27,4,5). Another reason which has been given is that Biblical Christians can confirm for themselves that the New Testament agrees with what the Old Testament taught and prophesied (Acts 17:11, Acts 26:22-23, Luke 24:44-48).
Given how many denominations and interpretations there are today how can you be sure that the early church got it right?
Biblical Christians are not to divide themselves into denominations (1 Corinthians 1:12-13). And Biblical Christians cannot interpret any one verse in the Bible in a way which contradicts what the Bible teaches as a whole. Biblical Christians can be certain that the early church got it right with regard to keeping the writings of the New Testament, for the reasons given above: the early church knew the writers of the New Testament to be eyewitnesses of Jesus Christ (2 Peter 1:16, 1 John 1:1-4, 1 Corinthians 9:1, John 19:35, John 21:24, Luke 24:48, Revelation 1:17-19) or their immediate followers (Luke 1:1-2, Hebrews 2:3). Also, the early church had received God's own Holy Spirit (1 Corinthians 2:11-13), Jesus Christ's own mind (1 Corinthians 2:16), so it was able to know whether a teaching of the writers was truly from Jesus Christ or not (John 10:27,4,5), just as Biblical Christians can still know this today for the same reason. And Biblical Christians today, just as the early church did, can confirm for themselves that the writers of the New Testament agree with what the Old Testament taught and prophesied (Acts 17:11, Acts 26:22-23, Luke 24:44-48).
For some examples of how the New Testament gospel was foretold in the Old Testament (Acts 26:22-23, 1 Corinthians 15:1-5, Luke 24:44-47), Christ's death for our sins was foretold in Isaiah 53 (cf. Acts 8:32-35, 1 Peter 2:24), Christ's crucifixion experience was foretold in Psalms 22 (cf. Matthew 27:46, Matthew 27:35), Christ not remaining dead was foretold in Psalms 16:10 (cf. Acts 2:27,31), Christ rising from the dead on the third day was foretold in Hosea 6:2 (cf. 1 Corinthians 15:4, Luke 24:46, Colossians 2:12), that Christ's New Covenant gospel (Matthew 26:28) would go forth to save both Jews and Gentiles (Acts 26:23b, Luke 24:47) was foretold in Isaiah 49:6 and Isaiah 42:6. Also, Moses foretold of Christ in Deuteronomy 18:15,18,19 (cf. Acts 3:22-24). Jesus fulfilled Zechariah 9:9 in Matthew 21:4-11. Psalms 118:22 was fulfilled at Jesus' first coming (Acts 4:10-12). Isaiah 7:14 was fulfilled by Jesus' virgin birth (Matthew 1:22-25). Isaiah 9:1-2 was fulfilled in Matthew 4:12-23. Psalms 110:4 is fulfilled by Jesus' high priesthood (Hebrews 6:20). Zechariah 6:11-12 and Zechariah 3:8-9 meant by type that the Christ would be a high priest named "Joshua"/"Jesus" ("Jesus" is simply the Greek form of the Hebrew name "Joshua"). "The Branch" is a title of the Christ (Isaiah 4:2-6, Jeremiah 33:14-17). And where it says "upon one stone shall be seven eyes" (Zechariah 3:9), even this is fulfilled by Jesus, the stone (1 Peter 2:7), who is at one point symbolically shown as having seven eyes (Revelation 5:6).