Why do Lutherans believe in Scripture alone when nowhere in the Bible it says that you should do that? Why not also believe in tradition like Catholics do?
We believe that Scripture alone is the source of our faith because that is what Scripture teaches. In the Bible God makes it clear that we are not to add to or subtract from his word (Deuteronomy 4:2; Revelation 22:18-19). The apostle Paul states clearly that God’s word is the foundation of the Christian Church and Jesus Christ is the chief cornerstone (Ephesians 2:20). There are no other foundations, like tradition, for our faith. Jesus tells us to search the Scriptures (John 5:39) and not to look anywhere else for saving truth (Luke 16:29). Jesus rebuked people who added to the word of God (Matthew 15:7-9).
Finally, Scripture alone is inspired and inerrant (1 Corinthians 2:13; 2 Timothy 3:16; 2 Peter 1:21). Unlike the words and writings of people, God’s word can always be trusted because he always speaks the truth (Numbers 23:19).