The Apostles' Creed is Trinitarian in structure with sections affirming belief in God the Father, Jesus Christ His Son and the Holy Spirit. The Apostles' Creed was based on Christian theological understanding of the Canonical gospels, the letters of the New Testament and to a lesser extent the Old Testament. Its basis appears to be the old Roman Creed known also as the Old Roman Symbol.
Because of the early origin of its original form, it does not address some Christological issues defined in the Nicene and other Christian Creeds. It thus says nothing explicitly about the divinity of either Jesus or the Holy Spirit. This makes it acceptable to many Arians and Unitarians. Nor does it address many other theological questions which became objects of dispute centuries later.
The earliest known mention of the expression "Apostles' Creed" occurs in a letter of AD 390 from a synod in Milan and may have been associated with the belief, widely accepted in the 4th century, that, under the inspiration of the Holy Spirit, each of the Twelve Apostles contributed an article to the twelve articles of the creed.
The word Symbolum, standing alone, appears around the middle of the third century in the correspondence of St. Cyprian and St. Firmilia, the latter in particular speaking of the Creed as the "Symbol of the Trinity", and recognizing it as an integral part of the rite of baptism.
The title Symbolum Apostolicum (Symbol or Creed of the Apostles) appears for the first time in a letter, probably written by Ambrose, from a Council in Milan to Pope Siricius in about AD 390 "Let them give credit to the Creed of the Apostles, which the Roman Church has always kept and preserved undefiled". But what existed at that time was not what is now known as the Apostles' Creed but a shorter statement of belief that, for instance, did not include the phrase "maker of heaven and earth", a phrase that may have been inserted only in the 7th century.
The account of the origin of this creed, the forerunner and principal source of the Apostles' Creed, as having been jointly created by the Apostles under the inspiration of the Holy Spirit, with each of the twelve contributing one of the articles, was already current at that time.