Everything that is taught word of mouth is also recorded in scripture and the point isn't to say there is one doctrine meant to be written and one to be spoken only. The point, always, is to say that the same doctrine is equally valid whether spoken or written. So if I or anyone read 1 Corinthians 11 aloud it would be the same validity as if it were read directly.
All scripture is given by inspiration of God. So in 1 Corinthians 11 is where it's recorded for us. The same goes for all doctrine of Christ, just insert the correct Biblical reference.
I'm not sure what exactly you mean. We all have the record that God gave of his Son. This is the one thing God exalted above all his name according to Psalm 138:2 and that on which the church was entrusted in Acts 20:32. That's what you must have to have life according to John 6:63 (not bread).
These are the true sayings of God. Not only that but they are eternally incorruptible, see 1 Peter 1:23-25 and Psalm 119:160. Also Proverbs 30:5-6, Isaiah 59:21, Matthew 24:35. The scripture cannot be broken according to John 10:35. The word shall not return void but shall accomplish its task according to Isaiah 55:11.
Not according to Matthew 16:18. But the spirit of antichrist was already present during the composition of the New Testament according to 1 John 2:18 and 1 John 4:3. They were already dealing with both Judaizers and gnostics who made up "oral traditions" after the manner of the Pharisees who were exposed in Mark 7:7-13 and are also prophesied against in places like 1 Timothy 4:1-3 and Galatians 6:13 and you had people making false accusations on the spot as we find in Acts 21:38 and Titus 1:10-11. Paul warned about them coming as soon as he would leave in Acts 20:29-31 right before entrusting them to God and his word in Acts 20:32. So its not like the state church of babylon was a total surprise when it arose. But fortunately despite all their efforts nobody was ever able to remove, change or replace the true word of God (that I'm referring to right now) and it remains as the best evidence to this day.
The question is always whether you were in it for God's word or not and whether you really do take it seriously, or you just write it off as something of lesser importance to manmade tradition, that can be ignored in favor of manmade traditions and new stuff that men come up with to keep you occupied. Forbidding to marry and commanding to abstain from meats come to mind, as does idolatry, later innovations to baptism, idolatry, and works gospel.
Beware lest any man spoil you through philosophy and vain deceit, after the tradition of men, after the rudiments of the world, and not after Christ. — Colossians 2:8
The whole doctrine is contrary to the Gospel.
But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe. — Galatians 3:22
For the promise, that he should be the heir of the world, was not to Abraham, or to his seed, through the law, but through the righteousness of faith. For if they which are of the law be heirs, faith is made void, and the promise made of none effect: — Romans 4:13-14
Yes there's that too. It really never ends the amount of complete 180 reversals they do of scripture. Turning it into a works gospel has absolutely got to be up there, as Galatians 1:8-9 makes clear. It can't be by grace and works if you believed Romans 11:6. At this point we could go to Romans 4:5, Ephesians 2:8-9, John 5:24, it really never ends.