It is a holy icon in which it shows the Virgin Mary breast feeding the baby Jesus. According to Yuri it's the progenitor of all the religious icons, according to the traditions of the Orthodox church, earning it the name 'the First Icon".
The story of the icon goes that after many hardships, Prince Vladimir finally converted the land of "Sary Su", also known as Tsaritsyn (now known as Volgograd, and once as Stalingrad). As a reward for his service, the church rewarded him with this icon.
Fool (Furu) states that some legends say that the Apostle Luke crafted the icon himself, and within it laid the keys to all the great secrets of the Christian faith. Sasha either didn't believe him, or didn't care, preferring he stuck with the facts when questioning the traitor Adept on the matter.
Yuudai Yamanobe hid the real icon in the Water Sanctuary, later recovering it before Friedrich Tanner (in Edgar's body) could retrieve it when using the Golden Noah to give himself another chance at claiming it.
Later on in the manga, the reason why the Adepts, the Theosis Project, and the Golden Tyrant are after it is revealed: it contains the Pneuma, or "Breath of Life" of Jesus Christ (basically his soul).
Using the High Ancient Circuit known as the David of Resurrection, the Golden Tyrant plans to use the Pneuma, as well as The Shroud said to have been wrapped around Jesus, and the Spear of Longinus for DNA samples, to clone Jesus Christ and start an artificial Second Coming.
Having united the Orthodox and Catholic churches for the first time in centuries, the Golden Tyrant would be provided the blood of Christ by the Catholics, and using his Theosis Project organization's Regen technology, all the Golden Tyrant would need is the David of Resurrection, which he ripped off of Sasha's flesh when confronting him on the Orient Express.