It has been debated for a while whether Guru Nanak ji visited Rome.
New evidence suggests that he did and indeed met Pope Leo X and a cardinal who would later become Pope Clement VII. Guru ji passed on the truth of God and God realisation and influenced by Guru ji, the Pope Clement VII and clergy later on would later speak against slavery (Africans being taken to the new worlds- americas etc) which would eventually lead to it's decline.
Apparently Papal documents refer to Nanacus but other documents which document key messages of Guru ji, Guru ji and entourage are referred to as 'Priests' as the Church could not afford to have it's name spoiled in the world (they were powerful at the time).
Older article with background (2010):
http://satguru.weebly.com/satguru-nanak-sahib-went-to-greece-rome-etc-according-to-forcible-entries-of-roman-empire1609.html
New evidence (March 2013)
http://satguru.weebly.com/