The Khan says "You men of God retreat, this boy advances..."

Kublai asked the "Latins" to bring him priests, but they all retreated (turned back or perished) from the arduous journey from Italy, which he says "many do not survive," whereas the "boy" did advance, he survived the journey when the men could not, "Our priests couldn't bear the rigors of the journey." Kublai says, "This says very little about your Savior." He wasn't poking fun at the Polos, it was irreverent humor at the expense of Christianity, soliciting laughter from his court, just before he says the Pope of Rome must bow before him and pledge his fealty to the Khan of Khans. Kublai was impressed that Marco survived the trip, and of course his expectations were exceeded when he recognized his talents as a poet, linguist, and overall intelligence.