Deuteronomy 6:4, "Hear, O Israel: Jehovah our God is one Jehovah: 5 and thou shalt love Jehovah thy God with all thy heart, and with all thy soul, and with all thy might. 6 And these words, which I command thee this day, shall be upon thy heart; 7 and thou shalt teach them diligently unto thy children, and shalt talk of them when thou sittest in thy house, and when thou walkest by the way, and when thou liest down, and when thou risest up." (ASV)
Jehovah has been worshiped as a single God who is a single person, uninterrupted, for 3,600 years by Israel. And yes, they know what the Hebrew word 'echad' means.