So the newest scriptures must mesh with the old. Or they should be set aside.
Your hard-and-fast rules need a little work. Maybe you're remembering that a prophet doesn't set aside the words of a previous prophet?
Exodus 21:24 eye for eye, tooth for tooth, hand for hand, foot for foot,
Matthew 5:38-39 Ye have heard that it hath been said, An eye for an eye, and a tooth for a tooth:
But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.
The reason for the changes
Matthew 19:7-8 They say unto Him, Why did Moses then command to give a writing of divorcement, and to put her away? He saith unto them,
Moses because of the hardness of your hearts suffered you to put away your wives: but from the beginning it was not so.
Matthew 5:19-20 Whosoever therefore shall break one of these least Commandments, and shall teach men so, he shall be called the least in the Kingdom of Heaven: but whosoever shall do and teach them, the same shall be called great in the Kingdom of Heaven.For I say unto you, That except
your righteousness shall exceed [that] of the scribes and Pharisees, ye shall in no case enter into the Kingdom of Heaven. (Matthew 23:27)
God does not trust just one writer with the fundamentals of His theology.
Yeah, He does.
Deuteronomy 18:15-19 The Lord thy God shall raise up to thee a prophet of thy brethren, like me; him shall ye hear: 16 according to all things which thou didst desire of the Lord thy God in Choreb in the day of the assembly, saying, We will not again hear the voice of the Lord thy God and we will not any more see this great fire, and so we shall not die. 17 And the Lord said to me, They have spoken rightly all that they have said to thee. 18 I will raise up to them a prophet of their brethren like thee, and I will put My words in His mouth, and He shall speak to them as I shall Command Him. 19 And whatever man shall not hearken to whatsoever words that prophet shall speak in My name, I will take vengeance on him.LXX
John 12:44-50 Jesus cried and said, He that believeth on Me, believeth not on Me, but on Him that sent Me. 45 And he that seeth Me seeth Him that sent Me. 46 I am come a light into the world, that whosoever believeth on Me should not abide in darkness. 47 And if any man hear My words and believe not, I judge him not: for I came not to judge the world, but to save the world. 48 He that rejecteth Me and receiveth not My words, hath one that judgeth him: the word that I have spoken, the same shall judge him in the last day. 49 For I have not spoken of Myself: but the Father which sent Me He gave Me a Commandment, what I should say and what I should speak. 50 And I know that His Commandment is Life Everlasting: whatsoever I speak therefore, even as the Father said unto Me, so I speak.KJV
Strictly speaking, it's not one writer but one speaker... with the two witnesses of Matthew and John. (Matthew 21:37)