The Luther rose was the seal that was designed for Martin Luther at the behest of Prince John Frederick, in 1530, while Luther was staying at the Coburg Fortress during the Diet of Augsburg. Luther wrote an explanation of the symbol to Lazarus Spengler: "a black cross in a heart, which retains its natural color, so that I myself would be reminded that faith in the Crucified saves us. 'For one who believes from the heart will be justified' (Romans 10:10)."