Because Apostolos knew the Turkish language, he took on the cause of his compatriots, who were being brutally taxed, and brought the matter personally to the government official in charge. During this encounter, he argued with great boldness for the rights of his countrymen. Because of this boldness, he was imprisoned.
St. Apostolos remained steadfast in his faith, and confessed bravely: "I am a Christian, and I will never leave my holy faith". They brought him from judge to judge, and each time, they would tempt him in various ways, bringing to mind his young age, offering to let him live in their [mansions]. For this reason, they offered him a gilded horse, that they might sweeten him with honors and good things. The Saint, however, replied to them: "I tell you again, as a precious Christian, whatever death you wish to inflict on me, by the grace of my Christ I will accept gladly. Therefore, don't drag on. Do you wish to burn me? I will gather the wood myself and prepare the fire. Do you wish to hang me? I will prepare the noose with my very own hands. Do you wish to behead me? Give me the sword to sharpen as much as is needed."
Having been tortured terribly and slandered, he was condemned to death. He received the crown of martyrdom by beheading on August 16th, 1686, in Constantinople (in front of the Yeni Tzami) , at the age of 19. After his martyrdom, those around saw a star descend from heaven, and remain above the holy relic of the Saint, and make the sign of the cross.
His relic was thrown into the Bosporos, while only his Precious Skull was saved by some Christians and given to the Patriarchate. In 1795 it was transferred to his hometown of Agios Lavrentios, and since then, his memory his honored in the region, and throughout Magnesia with special magnificence on August 16th, centered around his magnificent church (built at the place where his home was), with thousands of the faithful taking part from all around Greece.
(amateur translation/summary from: http://imd.gr/site/articles/top/topikoi_agioi/65, http://synaxarion.gr/gr/sid/390/sxsaintinfo.aspx, http://apantaortodoxias.blogspot.com/2010/05/blog-post_24.html)