Demetrius I (Greek: Δημήτριος Α`, born 185 B.C., reign 161-150 B.C.), surnamed Soter (Greek: Σωτήρ - "Savior"), was a ruler of the Hellenistic Seleucid Empire.
Demetrius was sent to Rome as a hostage during the reign of his father Seleucus IV Philopator and his mother Laodice IV . When his father was murdered by his finance minister Heliodorus in 175 B.C. , his uncle Antiochus IV Epiphanes killed the unsurper, but usurped the throne himself. He became the mad man who tried to eliminate Judaism all together but failed. . When Antiochus IV died in 163 B.C. , his 9-year-old boy son Antiochus V Eupator was made king by Lysias. Demetrius was then 22 years old (thus he was born in 185 B.C.). He requested the Roman Senate to restore the Syrian throne to him, but was rejected, since the Romans believed that Syria should be ruled by a boy rather than a man . Two years later, Antiochus V was greatly weakend because Rome sent an emmissary to sink his ships and hamstring his elephants for his violation of the Peace of Apamea, storing up too much weaponry. Demetrius escaped from confinement and was welcomed back on the Syrian throne in 161 B.C. . He immediately killed Antiochus V and Lysias.
Demetrius I is famous in Jewish history for his victory over the Maccabees, killing Judas Maccabaeus in Nisan, 160 B.C. . Demetrius acquired his surname of Soter, or Savior, from the Babylonians, whom he delivered from the tyranny of the Median satrap, Timarchus. Timarchus, who had distinguished himself by defending Media against the emergent Parthians, seems to have treated Demetrius' accession as an excuse to declare himself an independent king and extend his realm into Babylonia. His forces were however not enough for the legal Seleucid king: Demetrius defeated and killed Timarchus in 160 B.C., and dethroned Ariarathes, king of Cappadocia. The Seleucid empire was temporarily united again. Demetrius may had married his sister Laodice V, by whom he had three sons Demetrius II Nicator, Antiochus VII Sidetes and Antigonus.
Demetrius' downfall may be attributed to Heracleides, a surviving brother of the defeated rebel Timarchus, who championed the cause of Alexander Balas, a boy who claimed to be a natural son of Antiochus IV Epiphanes. Heracleides convinced the Roman Senate to support the young pretender against Demetrius I.
The Jews also had a role in the down fall of Demetrius I. Alexander Balas came with a mercenary army, landed and occupied Ptolemais, and reigned as a rival king of the Seleucids in 152 B.C. . He appointed Jonathan Maccabaeus, the borther and successor of Judas Maccabaeus, as the high priest of Judea, in order to made the jews an ally of his. Jonathan, who was born of a priestly family. but not from Zadok, the high priestly stock, took the title in Tishri, 152 B.C. . When Demetrius heard of it, he wrote a letter granting more priviledges to Jonathan (1 Macc. 10:25-45). The Jews did not believe in him, because of his past persecutions of the Jews. They join with Balas, who defeated and killed Demetrius I in 150 B.C. 
A Syrian, called Soter (Σωτήρ), or "the Preserver," the son of Seleucus Philopator, and sent by his father, at the age of twenty-three, as a hostage to Rome. He was living there in this condition when his father died of poison, B.C. 176. His uncle Antiochus Epiphanes thereupon usurped the throne, and was succeeded by Antiochus Eupator. Demetrius, meanwhile, having in vain endeavoured to interest the Senate in his behalf, secretly escaped from Rome, through the advice of Polybius the historian, and, finding a party in Syria ready to support his claims, defeated and put to death Eupator, and ascended the throne. He was subsequently acknowledged as king by the Romans. After this, he freed the Babylonians from the tyranny of Timarchus and Heraclides, and was honoured for this service with the title of Soter. At a subsequent period he sent his generals Nicanor and Bacchides into Iudaea, at the solicitation of Alcimus, the high-priest, who had usurped that office with the aid of Eupator. These two commanders ravaged the country, and Bacchides defeated and slew the celebrated Judas Maccabaeus. Demetrius at last became so hated by his own subjects, and an object of so much dislike, if not of fear, to the neighbouring princes, that they advocated the claims of Alexander Balas, and he fell in battle against this competitor for the crown after having reigned twelve years (from B.C. 162 to B.C. 150). His death was avenged, however, by his son and successor Demetrius Nicator (Just.xxxiv. 3Just., xxxv. 1).