The Imperial Diet of the Holy Roman Empire convened in the walled city of Worms in January of 1521. It was the first Diet where the newly crowned Emperor Charles V presided and he was anxious to deal with the issue of Martin Luther. Charles' initial reaction was to grant Luther an audience at the Diet and as early as November 1520 he had issued Luther an invitation but the Papal Legate Aleander advised against it and Charles withdrew the summons.
When the Diet convened for business Charles suggested to the 70 princes present that Luther be denounced as a heretic by Imperial Edict. The German princes refused to do this which Charles compelled to issue Luther with yet another invitation to the Diet. The summons was not for a hearing but rather to give Luther an opportunity to recant his position before the nobility of the German nation.
According to the constitution of the Holy Roman Empire, no German could be tried or outlawed for heresy or any other charge outside of Germany. Luther had already been excommunicated and condemned as a heretic by the Pope and the summons to the Diet of Worms was Charles V's attempt to make this legitimate.
Since Luther had been branded a heretic by Rome he had a significant bounty on his head and the emperor extended him a Safe-Conduct to ensure that he would arrive in Worms in one piece. An Imperial Safe-Conduct was essentially an armed escort with the Imperial insignia which declared to everyone watching that the person being escorted was under the protection of the Emperor.
Luther set out for Worms in April of 1521 in a little wagon, provided by the Wittenberg town council. He was accompanied by three of his closest friends. As they passed through towns and villages on their way to Worms, people flocked to catch a glimpse of the courageous Dr. Luther who had dared to defy the authority of Rome.
When Luther passed through his former stomping grounds in Erfurt, peopled lined the streets and climbed onto rooftops to catch a glimpse of him. Ironically he was invited to stay at the Augustinian cloister which was his former home and which had soundly denounced him as a heretic. He was even invited to preach after morning mass.
But Luther was also aware, that for all the cheering masses who encouraged him, his fate could very well resemble that of Jan Hus, who more than a hundred years ago had been promised a Safe-Conduct by Emperor Sigismund only to have it revoked. Hus was burned as a heretic by Sigismund, who ignored the Safe-Conduct and arrested him as soon as he entered Konstanz.
When they reached the town of Weimar, people began to cry out that Luther had already been condemned at Worms, along with his books. When he heard this, the imperial guard who was escorting Luther, Caspar Strum, turned to him and asked "Doctor, Sir are you sure you want to go?" To which Luther responded in the affirmative.
Just before he entered Worms, his friend Spalatin wrote to him on behalf of Prince Frederick, warning Luther to stay away because the situation in Worms was dangerous to which Luther replied "if there were as many devils in Worms as chimneys on the roofs, I would nevertheless come there"