Q: What do the scriptures principally teach?

A: The scriptures principally teach what man is to believe concerning God, and what duty God requires of man. (1)

1:

Micah 6:8. He hath shewed thee, O man, what is good; and what doth the Lord require of thee, but to do justly, and to love mercy, and to walk humbly with thy God?

John 20:31. But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.

John 3:16. For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.

(From the Westminster Shorter Catechism)