Armstrongism refers to the teachings and doctrines of Herbert W. Armstrong while leader of the Worldwide Church of God (WCG). His teachings are professed by him and his followers to be the restored true Gospel of the Bible. Armstrong said they were revealed to him by God during his study of the Bible. The term Armstrongite is sometimes used to refer to those that follow Armstrong's teachings. Armstrongism and Armstrongite are generally considered derogatory by those to whom it is applied, who prefer to be known as members of the Church of God (COG). These doctrines were also espoused by his sons Richard David Armstrong (until his death in 1958) and Garner Ted Armstrong (until his death in 2003) with slight variations.

Herbert Armstrong's teachings have similarities to those of the Millerites and Church of God (Seventh Day) (sometimes referred to as "COG7" to differentiate it from similarly styled sects named "Church of God" which worship on Sunday and generally hold to traditional Christian teachings), from which WCG is spiritually and organizationally descended. The religion is a blend of Christian fundamentalism, non-belief in the Trinity and some tenets of Judaism and Seventh-Day Sabbath doctrine. Armstrong himself had been a COG7 minister before the Oregon conference stripped him of his ministerial credentials and excommunicated him for his seeking to "water down" and change their long-established COG7 doctrines. It was in the fall of 1937 when Elder Armstrong's credentials were revoked by the Salem Church of God organization. The reason given by the Board of Twelve Oregon Conference of the Church of God, 7th Day (COG7) for this adverse action against Herbert W. Armstrong, was because he taught and kept the annual Feast days. But the real reason seems to have been because of his uncooperative attitude. Armstrong then began his own ministry.

Armstrong taught that most of the basic doctrines and teachings of mainstream Christianity were based on traditions, including absorbed pagan concepts and rituals (i.e. religious syncretism), rather than the Judeo-Christian Bible. His teachings have consequently been the source of much controversy. Shortly after Armstrong's death in 1986, the Worldwide Church of God started revising its core beliefs toward the concepts, doctrines, and creeds of mainstream Christianity. This resulted in many ministers and members leaving the WCG to start or join other churches, many of which continue to believe and teach Armstrong's doctrines to one degree or another. In 2009, the WCG changed its name to Grace Communion International (GCI). Today, the official doctrinal position of GCI is mainstream evangelical, although there are still GCI ministers and members who do not fully embrace all of the changes.

Some of Armstrong's identifiable doctrines are in addition to or are different from traditional mainstream Christian doctrines. Many groups and churches which splintered in the aftermath of doctrinal changes within the Worldwide Church of God continue to hold many or all of these teachings of Armstrong.

The God Family doctrine holds that the Godhead is not limited to God (the Creator) alone, or even to a trinitarian God, but is a divine family into which every human who ever lived may be spiritually born, through a master plan being enacted in stages. The Godhead now temporarily consists of two co-eternal individuals (see Binitarianism), Jesus the Messiah, as the creator and spokesman (The Word or Logos), and God the Father.

According to this doctrine, humans who are called by God's Holy Spirit to repentance, who [accept], hope to inherit, the gift of eternal life made possible by Jesus' sacrifice, who commit to live by "every word of God" (i.e. biblical scripture), and who "endure to the end" (i.e. remain faithful to live according to God's way of life until either the end of their own lifetime or the second coming of Jesus) would, at Jesus' return, be "born again" into the family of God as the literal spiritual offspring or children of God. Armstrong drew parallels between every stage of human reproduction and this spiritual reproduction. He often stated that "God is reproducing after his own kind— children in his own image." Whatever the changes brought about by this new entrance of humans into God's family, God the Father will always be the omnipotent sovereign and sustainer of both the universe and the spiritual realm, forever to be worshipped as God by the children of God. Jesus, as the creator of the universe and the savior of God's children, will always rule the Kingdom of God, which will ultimately grow to fill the entire universe, and he likewise will forever be worshipped as God by the children of God.

Armstrong taught that the Bible (excluding the Biblical apocrypha and the deuterocanonical books) is the authoritative Word of God (The Proof of the Bible). He taught that even though the Bible's message is inerrant, it had been distorted as the result of many conflicting interpretations of it, and the Gospel's full message of the Kingdom of God as it was understood by the original apostles was not restored until the 20th century, when God opened Armstrong's mind to the plain truth of scripture and revealed the Gospel's full message of the Kingdom of God to the Church through him (Armstrong). Armstrong taught that all other churches which called themselves "Christian" churches were not merely apostate churches, they were actually counterfeit churches because their histories could be traced back to the first century, and they are also described as false churches in the epistles (which refer to a "false gospel", "false ministers", and "false apostles"), the eighth chapter of the Book of Acts (the appropriation of "Christian" trappings by influential and ambitious pagan religious figures [including a man known to secular history, Simon Magus, mentioned in Acts]) and later historians like Eusebius.

The observance of the Sabbath from dusk on Friday to dusk on Saturday was the first non-traditional religious practice (as compared to mainstream Christianity). In several of his books, Armstrong wrote that after his wife Loma met a member of a Sabbatarian church group (the Church of God (Seventh Day)), she challenged him to use scripture to prove that Sunday was the proper day for Christian worship, as Herbert claimed. After months of Bible study, Armstrong concluded that there was no sound scriptural basis for Christian worship on Sunday, instead, he asserted that for decades after the establishment of the Church age, the Apostles and the first generation of Christians, both Jewish and Gentile converts, continued to set an example for all Christians by observing the Sabbath on the seventh day of the week (from Friday at sunset to Saturday at sunset).