John 16:13 I believe Holy Spirit inspired the writings that we call scripture and preserved those writings throughout history.
As the Apostolic Fathers began to pass, the early church collected their writings to preserve them. In the case of Luke, Acts, Mark, these writings were very early accepted as scripture because of the close affiliation between Luke and Paul on the one hand and Mark and Peter on the other.
The Jewish scriptures (Old Testament for us) had long been debated and established within the Jewish community and was accepted by the early church as scripture, including David's "diary" as you call it.
Everything we have as scripture was collected and preserved by early churches by the end of the first century.
When new writings began to appear in the second century and beyond, the churches looked to the writings already preserved to be the measuring rod "canon" by which to judge those new writings as authentic or discard them as not scripture.
I have stated clearly what I believe. I have not held back claiming to have a "pure mind" one the one hand but only being "clear enough" on the other. I am not ashamed of what I believe.
Our Lord Jesus Christ has clarified these issues for me, through indwelling Holy Spirit, who leads me and all He indwells to an understanding of the truth of all issues.
Not that I have already become perfect in my understanding, but I work out my salvation in fear and trembling as should we all.
I am always ready to learn from a brother or sister in Christ who can speak plainly, showing me by scripture how I have misunderstood God's Word.
Are you able to let your yes be yes and your no be no? Do you believe Luke, Acts, Mark are scripture?