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.

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.

Westminster Project - For more info follow this link to a lesson from Thomas Boston