Thousands of people cheered when Barack Obama took to the stage in front of the Brandenburg Gate on Thursday. The former US president participated in a public discussion that was part of the Protestant "Kirchentag," or Church Day, in Germany's capital.

At the event celebrating the 500th anniversary of the Reformation, Obama spoke with German Chancellor Angela Merkel about how to "shape democracy and take responsibility at home and in the world." Some of Merkel's political opponents have criticzed the event as a publicity stunt set up to boost her popularity ahead of Germany's parliamentary elections in September.

Obama is a lot more popular in Germany than his successor Donald Trump. The thousands of visitors who wanted to see the former president had to go through tight security checks. In addition to police on the ground, helicopters patrolled the skies and snipers were camped out on the roofs of nearby buildings.

Obama praised Merkel as a leader who had done "outstanding work." He went on to talk about the rise of nationalism and xenophobia in many western countries and said that "we have to push back against those trends that would violate human rights or suppress democracy or restrict individual freedoms."

Obama also highlighted the need for liberal democracy and diplomacy. Members of the audience held up signs that made it clear they would have preferred a third Obama term to Trump's current policies. While the former president did not refer to the current one directly, he said to the cheering crowd that "we can't isolate ourselves. We can't hide behind a wall."

The discussion between Obama and Merkel was hosted by Kirchentag President Christina Aus (pictured). Obama has said in previous years that he considered himself a "devout Christian." When discussing refugee policies on Thursday, he said that a child on the other side of a border deserved just as much love as his own in the eyes of God.

The Kirchentag marks Obama's eighth visit to Germany. His first to Berlin took place during his first presidential race. In July 2008, the then-presidential candidate gave a foreign policy speech at the Victory Column in front of an enthusiastic crowd of 200,000 people. Long before anyone had ever heard of a US surveillance scandal, many young Germans were enamored with Obama.

Did Luther really nail his 95 theses to the main door of the Wittenberg Castle church? Reformation historians are still discussing this point 500 years later. In fact, Luther himself never mentioned the theses. Belgian historical painter Ferdinand Pauwels didn't seem to care - he painted Luther with hammer and nails anyway.

"Exsurge Domine" ("Arise, Oh Lord") was a papal bull written by Leo X on June 15, 1520. In it, he asked Martin Luther to recant what he had written in his 95 theses, or else face expulsion from the church. The reformer would not back down and burned the paper in an act of protest.

Hero or ruffian? The German image of Martin Luther has changed throughout history depending on the political climate. The Nazis, as well as the communist East German regime, used the reformer to suit their own purposes. This memorial, erected in 1895 in Eisenach, recalls Luther's time at the Wartburg Castle. There, he translated the New Testament of the Bible into German in only 10 weeks.

The religious divisions from the 16th to 18th centuries affected artists as well. The Protestant artwork of Lucas Cranach, the Elder, placed the pope with lewd figures. The Catholic side countered, for example, with Egbert II van Heemskerck's painting "Luther in Hell." The reformer seems to be surrounded with eerie monsters, hellish demons and devils at the entrance to hell.

Adam and Eve are depicted eating the forbidden fruit from the Tree of Knowledge. Written about in Christian teachings, this "sinful" topic was taken up not only by theologians, philosophers and worshipers, but also by artists. This baptismal bowl from the first half of the 16th century is one of the highlights of the exhibition in Wartburg.

Lucas Cranach, the Elder (1472-1553), was not only a well-known painter of the German Renaissance. He was also a painter of the Reformation and a good friend of Martin Luther's The artistic entrepreneur had a workshop for painting and graphic printing in Wittenberg. In this picture, he depicts Jesus Christ with a group of women who are holding their children up to him to be blessed.

Lucas Cranach painted "Bildnis des Hans Luther" (Luther's father) in 1527. Many portraits of the reformer came from his studio. Today, they are regarded as masterpieces of art history. In these depictions, Luther sometimes resembles a rigid monk, or as his bearded alias "Junker Jörg," or again as a scholar. Regardless of how he was depicted, Cranach's paintings gave a face to the Reformation.