Hi, Scient Artist Pagan

I am not familiar with artifacts and historical studies on this. I do understand that humans in general can favor the artifacts and historical studies which fit with each person's own prejudices; and I am human, too; so surely I can do this, also.
But when I read that a Gospel or New Testament letter was written at a certain time, I am curious how they decided what that writing time was.
For example, if someone says a Gospel was written in 70 AD, why? If they go by writing style, of a certain cultural time, fine. But it could have been translated from an earlier writing with a style update, for all I know, like how Bible versions are done, today, with updating of writing style and vocabulary. And earlier copies of earlier versions could have worn out with use. The earlier versions could have been in Hebrew or Aramaic, for all I know, and before they were worn out were translated into Greek manuscripts which were discovered.
Our Apostle Paul says,
"I have become all things to all men, that I might by all means save some," in 1 Corinthians 9:22; so from this I see how Paul might adjust how he lived and communicated, in order to reach people in their cultural settings; and so, like this, possibly early manuscripts were translated to adjust to the writing style and vocabulary of whoever would be using those manuscripts. Therefore, in case ones date when something was written, based only on the style of the earliest manuscript found, this could be "hasty", I think.
Also, ones could have been sharing pages or portions of manuscripts they held, so that only partial copies were discovered, in some cases; and so, ones would suppose that parts were "added", later, if older copies were found without those parts. Ones can interpret according to how they have been trained to interpret, and/or more or less according to their motives and character, then, I offer.
If Jesus died when He was 30, His mother Mary was still alive and at His cross > John 19:26-27 > so, she was older than 30. Plus, John lived long enough after Jesus was on this earth, so that John had time to be put on the prison island of Patmos > Revelation 1:9. So, in case John was Jesus' age, he would have lived longer than Jesus; Jesus trusted John to take care of His mother Mary.
Now you might have claims of information which you can use to question things, but I am aware how people in this world can rig things, and fix which books and historical claims make it to the present day. I believe we have certain people claiming to be "Christian", too, who can rig things, maybe in order to get a name for themselves.
So, I pray and trust God to do with me whatsoever He pleases. And I find He does better with me and others, than how I can understand the Bible to mean, with me and others. I might be Bible smart, but very love stupid; but God is able to correct us into real loving.
Your own character and your own motives will have a lot to do with what you decide, how you see things, and how you are able to interpret and filter what you have for claims of information. I am finding that as God corrects me, I become more able to understand the Bible and life more in terms of enjoying and trusting God while I have compassion for any and all people and share more and more as family with His people. But, as for those who are deceivers >
"Destruction and misery are in their ways;
. The way of peace they have not known."
. . . . . . . . . . . . . . . . .(Romans 3:16-17)
You "might" take a look at how people are becoming while they do what they are doing, and how their marriages are becoming. God's grace is almighty, and His love can effect how our minds and bodies function, so that we live longer than we would by living for ourselves in stress and using each other. Love does not have us using anyone, and
"Love does no harm to a neighbor" (in Romans 13:10). So, yes, obviously there are people who have claimed to be Christians, but who have harmed people. So, it is good to make sure; and making sure with God Himself works best, I will offer you
The Bible itself does say >
"Test all things; hold fast what is good." (1 Thessalonians 5:21)
But this means to test with God and the ability and honesty of love which you can have with Him.
"'Take My yoke upon you and learn from Me, for I am gentle and lowly in heart, and you will find rest for your souls.'" (Matthew 11:29)