On: every psalm and song in the daily prayers uses today’s English. Off: all of them use the older “thee and thou”. Until you choose, the Songs of Zechariah, Mary and Simeon use today’s English, as most churches sing them. The Lord’s Prayer and the prayers for the dying show the form most people learned.
The letters attributed to Paul. Romans and Galatians are the two the Reformation leaned on hardest, since both argue that a person is justified by faith rather than by works of the law.
In Scripture — The canon, translation, and how Lutherans read the Bible.