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.
One God in three persons: Father, Son, and Holy Spirit. Confessed in the Apostles', Nicene, and Athanasian Creeds, all three of which the Book of Concord includes.
In Doctrine — What Lutherans teach, and where it differs from neighbouring traditions.