I am the LORD your God, who brought you out of the land of Egypt, out of the house of slavery. (Exodus 20:2)
In declaring Who He is, G-d expects us to know there is a G-d and it is Him.
Messiah affirms the commandment "know there is a G-d" implicitly by proclaiming that we should not only know about G-d but we should know G-d:
This is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent. (John 17:3)
Why are we to know there is a G-d? Because Messiah knows Father G-d and we are to be like Messiah.
All things have been handed over to Me by My Father; and no one knows the Son except the Father; nor does anyone know the Father except the Son, and anyone to whom the Son wills to reveal Him. (Matthew 11:27)
Messiah tells us that He will reveal the Father to anyone whom He wills.
Messiah obeyed the commandment "know there is a G-d" by not only knowing about G-d but by intimately knowing the Father and living according to all of the commandments (John 14:31).
It is only through Messiah that we can know G-d.
The traditional observance of this commandment is not just to "know there is a G-d" (as if someone would tell us about G-d and then we could say "Oh. Now I know.") Observance of this commandment is that a person's entire life should be lived according to the knowledge that there is a G-d. Everything we think, say, and do should occur with that knowledge of G-d front and center. It should shape everything we are and do.
The essence of this commandment can be found in the statement "to know and not to do is not yet to know."
To know there is a G-d and yet not to live life according to that knowledge is not yet to know.
We are able to fulfill this commandment today and should know there is a G-d.
This commandment is given in Exodus 20:2 as G-d is speaking the words of His covenant to Israel. It is as if G-d is introducing Himself to the entire nation: "Hello. I'd like to introduce myself. I am the LORD your G-d, who brought you out of the land of Egypt. Nice to meet you. Let me show you around."
All the World
This commandment is applicable to both Israel and the foreigner... indeed the whole world. As it is written of King David when he said:
"This day the LORD will deliver you up into my hands, and I will strike you down and remove your head from you. And I will give the dead bodies of the army of the Philistines this day to the birds of the sky and the wild beasts of the earth, that all the earth may know that there is a God in Israel, and that all this assembly may know that the LORD does not deliver by sword or by spear; for the battle is the LORD'S and He will give you into our hands." (1 Samuel 17:46)
And the prophet Isaiah declares:
Turn to Me and be saved, all the ends of the earth; For I am God, and there is no other. I have sworn by Myself, The word has gone forth from My mouth in righteousness And will not turn back, That to Me every knee will bow, every tongue will swear allegiance. (Isaiah 45:22-23)