Both ὁ θεός and τον θεόν are translated as "The God" in English.
ὁ θεός = the God (Nominative Case)
τον θεόν = the God (Accusative Case)
Nominative Case is used as a Subject in a sentence. (It indicates the person who acts).
Ο θεός αγαπάει τους ανθρώπους. = (The) God loves (the) people.
Accusative Case is used as an object in a sentence. (It indicates the person who receives the action.)
"Αγαπάω τον Θεό." = I love (the) God.
"a God" in Greek is "ένας Θεός"