Did Christianity ' adopt from Judaism a belief in the existence of a single God ' or did true Christianity continue that which had long been established as truth, that God is one person, not "three persons in a Godhead" ? Upon being asked what the greatest commandment is, Jesus responded: "The first is, ' Hear, O Israel, Jehovah our God is one Jehovah, and you must love Jehovah your God with your whole heart and with your whole soul and with your whole mind and with your whole strength".(Mark 12:28-30)
Jesus was quoting from Deuteronomy 6:4, 5, spoken by Moses to the nation of Israel some 40 years after the Israelites had been freed from Egyptian bondage in 1513 B.C.E. The Egyptians believed in a multitude of triad gods, such as Osiris, Isis, and Horus, a father-mother-son triad.
Ancient Egypt was rife with polytheism, for every city and town had its own local deity, bearing the title "Lord of the City". In the tomb of Thutmose III was found a list of the names of some 740 gods. The Israelites needed to understand that God is "one Jehovah", not a triad, as well as one in purpose.
Jesus, though, knew that true Christianity that he established would become corrupted after the death of his appointed apostles who acted as a "restraint" to the tide of apostasy that was already beginning while they were alive, showing this through the illustration of "the wheat and weeds" at Matthew 13:24-30, 36-43.(see also 2 Thess 2:3-6 where the apostle Paul describes the coming apostasy before "the day of Jehovah" arrives)
Among the falsehoods that this foretold apostasy began to promote is that God is a trinity (Father, Son and Holy Spirit), three "persons" co-equal and co-eternal. It basically began with Tertullian (about 160-230 C.E.), who used the Latin word trinitas, proposing that God is "one substance consisting in three persons", though he said: "How can it be that anything, except the Father, should be older, and on this account indeed nobler, than the Son of God, the only-begotten and first-begotten Word ?"
Later writers used his ideas to build toward the Trinity doctrine, instead of adhering to Scripture, such as Jesus words to his apostles that "the Father is greater than I am".(John 14:28) Or that of the Father saying at Jesus baptism: "This is my Son, the beloved, whom I have approved".(Matt 3:16)
Sincere and serious Bible students can easily recognize that if the Father and Son were equal, the Father would not refer to Jesus as "my beloved, whom I have approved". Jesus would not need the Father's approval if he were God. Jesus would also not be called "my servant whom I (God) chose, my beloved, whom I have approved" at Matthew 12:18 and in which Matthew was quoting from Isaiah 42:1.
Jesus succinctly describes who he is, saying: "For God loved the world so much that he gave his only-begotten Son, so that everyone exercising faith in him might not be destroyed but have everlasting life".(John 3:16) Jesus established that he was "begotten", created by "the Father".(see Rev 3:14 where Jesus calls himself "the beginning of the creation by God")
And in using the wording of "only-begotten" showed that he is unique in being created directly by God, for these words are from the Greek monogenes that means "single of its kind, only".(Thayer's Greek-English Lexicon, see also Luke 7:12 where monogenes [only-begotten] is used with regard to a widow who had only one son as well as at Judges 11:34 [in the Greek Septuagint] with regard to Jepthah's "one and only child")
After the death of the apostles who acted as a restraint from apostasy (2 Thess 2:6), a plethora of false religious ideas were accepted within what later came to be called Christendom, such as the trinity, which elevates Jesus to the status of God as well as the holy spirit, which is not a person but God's active force.(see Ps 104:29, 30 that shows that the word "spirit" (Hebrew ruach, "breath", KJV) is an invisible active force that when taken away, a person dies and that when God sends his spirit (Hebrew ruach) forth, "they are created", renewing the ground, hence being an invisible active force that produces visible results)