Deuteronomy 6:4 "Hear, O Israel: The LORD our God, the LORD is one!" (NKJV)
The Hebrew word for "one" is interesting. It is "ECHAD", always used in connection with a collective body. "ECHAD" means a compound unity and would be used in such an expression as "one cluster of grapes." Deuteronomy 6:4 "Jehovah our ELOHIM is a united Jehovah." The great teaching here is a unity trinity. Three Persons make up the Godhead, yet they constitute one essence. Note other examples of the use of the word ECHED, one in unity. (Genesis 2:24; 3:22; 11:6; John 17:21-22 )
Isaiah 44:6 "Thus says the LORD, the King of Israel, And his Redeemer, the LORD of hosts: 'I am the First and I am the Last; Besides Me there is no God.'" (NKJV)
Israel's God is the one and only God. There is no God apart from the one God revealed in three Persons, one divine essence.
Isaiah 43:10 "You are My witnesses," says the LORD, "And My servant whom I have chosen, That you may know and believe Me, And understand that I am He. Before Me there was no God formed, Nor shall there be after Me. I, even I, am the LORD, And besides Me there is no savior." (NKJV)
Israel was chosen to know the true God; to believe the true God; to understand that there was no other God other than the true God, and that the true and living God is the only Saviour. There is only one God and none beside Him (Deuteronomy 4:35 ). God has specially spoken to Israel (Deuteronomy 4:33 ), and shown them His great power, (Deuteronomy 4:34 ), that they might know the true God.