Deuteronomy 6:4 says, "Hear, O Israel! Yahweh is our God, Yahweh is one [echad]!"
The Hebrew word "echad" is used most often as a unified one, and sometimes as numeric oneness. For example, when God said in Genesis 2:24 "the two shall become one [echad] flesh," it is the same word for "one" that was used in Deut. 6:4.
A different word, "yachid," the main Hebrew word for solitary oneness, indisputably means an absolute numeric one. Anti-Trinitarians would naturally expect such a word to be commonly used of God, but it is never used to describe God.
Add to this plural pronouns like: "let US make man in OUR image" and Trinitarians have irrefutable evidence of the Trinity in the Old Testament.
We find exactly the same in the New Testament as we do in the Old, namely, a combination of words meaning unified versus numeric oneness being employed to describe God's oneness. While yachid is never used of God’s oneness in the Old Testament, the corresponding Greek word "mono" is used of God’s oneness in the New Testament. But this is exactly what Trinitarians would expect to be the case because there are three persons in the one God.
Perhaps one of the strongest arguments the Trinitarian can use in the discussion of the words "echad" and "yachid" is the fact that Jews, shortly after the rise of Christianity, removed "echad" from Deut. 6:4 and added in its place the word "yachid". If the use of "echad" instead of "yachid" in Deut. 6:4 gave no help to the early Christians in proving to the Jews that Yahweh of the Old Testament was a multi-personal God, then Jews would not have felt compelled to change the word. If it is really that insignificant, then they would have told us the argument Christians were using to prove the Trinity is invalid to native Hebrews who know and speak the language. But instead, the Jews responded by changing the word in Deut. 6:4 from the unified oneness (echad) to the numeric oneness (yachid).
Jesus quoted Deut 6:4 in Mk 12:29 and chose the "unified oneness" word "hen" which is the same word used by Jesus in Mt 19:5, "the two shall become one (hen) flesh. It is significant that Jesus did not use "mono" in Mk 12:29. The word "hen" directly corresponds to "echad" which was used in Deut 6:4. Both texts used "unified oneness" words rather than absolute numeric oneness to the exclusion of all others.
Five different words for "one" in the Bible:
Echad (OT) - (Unified one: Gen. 2:24; Deut. 6:4) (Absolute numeric one: Ezekiel 33:24) - Used of God’s oneness
Yachid (OT) - (Always absolute numeric one: Judges 11:34) - Never used of God’s oneness
Bad (OT) - (Absolute numeric one: Isaiah 37:20) - Used of God’s oneness
Hen (NT) - (Unified one: John 10:30; Matthew 19:5; Mk 12:29) (Absolute numeric one: Galatians 3:20) - Used of God’s oneness
Monos (NT) - (Absolute numeric one: Matthew 24:36; 1 Timothy 1:17) - Used of God’s oneness
Following are a number of Old Testament passages that are either directly quoted in the New Testament, or contain parallel thoughts:
Two shall become one flesh (Echad, Old Testament: Gen. 2:24) (Matt. 19:5)
God is one (Echad, Old Testament: Deut. 6:4) (Hen, New Testament: Mark 12:29,32; John 10:30; 1 Cor. 8:4; Eph. 4:6)
One people (Echad, Old Testament: Gen. 11:6; Gen. 34:16; Gen. 22) (Hen, New Testament: John 11:52; Gal. 3:28)
One heart (Echad, Old Testament: 2 Chron. 30:12; Jer. 32:39) (Hen, New Testament: Acts 4:32; Phil. 1:27; Phil. 2:2)
Two objects becoming one (Echad, Old Testament: sticks: Ezek. 37:17) (Hen, New Testament: flocks: John 10:16)
Assembly as one (Echad, Old Testament: Ezra 2:64) (Hen, New Testament: Romans 12:5, 15:6; 1 Cor. 12:5,12)
(the above information is from: Bible.ca)