The writers of the Bible wrote as they were inspired by the Holy Spirit:

(Jude 1:3 NIV) Dear friends, although I was very eager to write to you about the salvation we share, I felt I had to write and urge you to contend for the faith that was once for all entrusted to the saints.

(2 Peter 1:21 NIV) For prophecy never had its origin in the will of man, but men spoke from God as they were carried along by the Holy Spirit.

The New Testament was written almost exclusively by eyewitnesses and those who talked to eyewitnesses. The only possible exception is Hebrews. Nobody knows for sure who wrote it.

(Luke 1:1-4 NIV) Many have undertaken to draw up an account of the things that have been fulfilled among us, {2} just as they were handed down to us by those who from the first were eyewitnesses and servants of the word. {3} Therefore, since I myself have carefully investigated everything from the beginning, it seemed good also to me to write an orderly account for you, most excellent Theophilus, {4} so that you may know the certainty of the things you have been taught.

(2 Peter 1:16 NIV) We did not follow cleverly invented stories when we told you about the power and coming of our Lord Jesus Christ, but we were eyewitnesses of his majesty.