Beside the obvious theological difficulties with the Oneness position, its main offence is that as it allows for only one 'being' to be in operation at any one period of time, this means that it inadvertantly ends up denying the person of both Jesus and the Holy Spirit when the Father is supposed to be evident.
Before the Nicene Creed: Since Christianity is a religion derived from Judaism, it should not be a surprise that from the beginning it has been adamantly monotheistic.
The early 2nd century Apologists like Justin Martyr rejected Marcionite and Gnostic dualism wholeheartedly and asserted the oneness of God in strong, almost Platonistic language.
Christ himself revealed the coming Spirit of God who also possessed divine attributes and too was spoken of as distinct from the Father. These statements of Jesus, along with the teaching of the epistles of Paul and John, presented a unique theological challenge to the early Christians. Speaking of Trinitarianism in the ante-Nicene period is somewhat anachronistic, since the word Trinity (Lat. trinitas) was first coined by the Latin father Tertullian in the 2nd century,
and the Trinitarian doctrine was not solidified as dogma until the early 4th century. However, Roger Olson reminds us that
"Christian belief in God as triune did not arise in the fourth century with Roman emporer Constantine and the Christian bishops that he dominated.
Belief that it arose then as part of a vague paganizing or Hellenizing of Christianity is a caricature often promoted . Roger Olson, ]u]
The Mosaic of Christian Belief,[/u] p. 135
He created the world by the word of His mouth (Ps. xxxiii. 6, 9). Natural sequences are His work (Jer. v. 22, 24; Ps. lxxiv. 15-17). He maintains the order of nature (Ps. cxlvii. 8-9, 16-18; Neh. ix. 6). He does not need the offerings of men, because "the earth is the Lord's and the fulness thereof" (Ps. xxiv. 1, 4, 7-13; compare Isa. i. 11; Jer. vii. 21-23; Micah vi. 6-8).
Nothing is affirmed of His substantial nature. The phrase "spirit of God" ("rua? Elohim") merely describes the divine energy, and is not to be taken as equivalent to the phrase "God is a spirit," viz., an assertion concerning His incorporeality (Zech. iv. 6; Num. xiv. 22; Isa. xl. 13). He can not, however, be likened to any thing (Ex. xx. 4-5; Isa. xl. 18) or to any person (Jer. x. 6-7). No form is seen when God speaks (Deut. iv. 15)
http://www.theopedia.com/Elohim
Elohim is plural for angels, whom GOD commanded to help with the creation, not a mortal man like Jesus. Isa 54:16 Lo, I--I have prepared an artisan, Blowing on a fire of coals, And bringing out an instrument for his work, And I have prepared a destroyer [angel of the Lord] to destroy.
Exo 14:19 And
the angel of God, which went before the camp of Israel, removed and went behind them; and the pillar of the cloud went from before their face, and stood behind them:
Num 22:31 Then the LORD opened the eyes of Balaam, and he saw the
angel of the LORD standing in the way, and his sword drawn in his hand: and he bowed down his head, and fell flat on his face.
He was only human when God created him as a mortal, without immortality. Anyone who has immortality cannot die, as Jesus did as a mortal. People do not have immortality in the second death.
2Th 1:9 who shall suffer justice--destruction age-during--from the face of the Lord, and from the glory of his strength,