Thank you for the question about God, “What is “God” and why did God reveal Himself in the First Commandment as thy “God”?”
The Westminister Confession is a great statement on the nature of God. It is quite complete in its definition and description of God. For your consideration, I quote the following:
“There is but one only,(a) living, and true God:(b) who is infinite in being and perfection,(c) a most pure spirit,(d) invisible,(e) without body, parts,(f) or passions,(g) immutable,(h) immense,(i) eternal,(k) incomprehensible,(l) almighty,(m) most wise,(n) most holy,(o) most free,(p) most absolute,(q) working all things according to the counsel of His own immutable and most righteous will,(r) for His own glory;(s) most loving,(t) gracious, merciful, long-suffering, abundant in goodness and truth, forgiving iniquity, transgression, and sin;(u) the rewarder of them that diligently seek Him;(w) and withal, most just and terrible in His judgments,(x) hating all sin,(y) and who will by no means clear the guilty.(z)”
(a) Deut. 6:4; I Cor. 8:4, 6. (b) I Thess. 1:9; Jer. 10:10. (c) Job 11:7, 8, 9; Job 26:14.
(d) John 4:24. (e) I Tim. 1:17. (f) Deut. 4:15, 16; John 4:24, with Luke 24:39.
(g) Acts 14:11, 15. (h) James 1:17; Mal. 3:6. (i) I Kings 8:27; Jer. 23:23, 24.
(k) Ps. 90:2; I Tim. 1:17. (l) Ps. 145:3. (m) Gen. 17:1; Rev. 4:8. (n) Rom. 16:27.
(o) Isa. 6:3; Rev. 4:8. (p) Ps. 115:3. (q) Exod. 3:14. (r) Eph. 1:11. (s) Prov. 16:4; Rom. 11:36. (t) I John 4:8, 16. (u) Exod. 34:6, 7. (w) Heb. 11:6. (x) Neh. 9:32, 33.
(y) Ps. 5:5, 6. (z) Nah. 1:2, 3; Exod. 34:7.
II. God hath all life,(a) glory,(b) goodness,(c) blessedness,(d) in and of Himself; and is alone in and unto Himself all-sufficient, not standing in need of any creatures which He hath made,(e) nor deriving any glory from them,(f) but only manifesting His own glory in, by, unto, and upon them: He is the alone fountain of all being, of whom, through whom, and to whom are all things;(g) and hath most sovereign dominion over them, to do by them, for them, or upon them whatsoever Himself pleaseth.(h) In His sight all things are open and manifest;(i) His knowledge is infinite, infallible, and independent upon the creature,(k) so as nothing is to Him contingent, or uncertain.(l) He is most holy in all His counsels, in all His works, and in all His commands.(m) To Him is due from angels and men, and every other creature, whatsoever worship, service, or obedience He is pleased to require of them.(n)
(a) John 5:26. (b) Acts 7:2. (c) Ps. 119:68. (d) I Tim. 6:15; Rom. 9:5. (e) Acts 17:24, 25.
(f) Job 22:2, 3. (g) Rom 11:36. (h) Rev. 4:11; I Tim. 6:15; Dan. 4:25, 35. (i) Heb. 4:13.
(k) Rom. 11:33, 34; Ps. 147:5. (l) Acts 15:18; Ezek. 11:5. (m) Ps. 145:17; Rom. 7:12.
(n) Rev. 5:12, 13, 14.
III. In the unity of the Godhead there be three persons, of one substance, power, and eternity; God the Father, God the Son, and God the Holy Ghost.(o) The Father is of none, neither begotten, nor proceeding: the Son is eternally begotten of the Father:(p) the Holy Ghost eternally proceeding from the Father and the Son.(q)
(o) I John 5:7; Matt. 3:16, 17; Matt. 28:19; II Cor. 13:14. (p) John 1:14, 18. (q) John 15:26; Gal. 4:6.” (Trustworthy Bible Teaching and Discipleship Resources).
I trust the Westminister Confession of Faith helps you to understand what Christians believe about the nature of God. I found the Confession to be very thorough and reflects the Scriptures accurately.
As to the reason why God revealed himself to the Israelites as God is that perhaps some enslaved Hebrews may have adopted a polytheistic view while living in Egypt for 400 years. God reveals to them that he is the Creator God and he is their Deliverer. As the Deliverer, the Israelites had the choice to assent to YHWH as their God or not.
Joshua said this in Joshua 24:15 in the NIV, “But if serving the LORD seems undesirable to you, then choose for yourselves this day whom you will serve, whether the gods your ancestors served beyond the Euphrates, or the gods of the Amorites, in whose land you are living. But as for me and my household, we will serve the LORD.”
Each generation has to make that choice whether s/he believes in the Creator God, YHWH or reject him as one’s God. They could choose to leave the Promise Land and go to another land to believe in that countries god(s) or goddess whether Egypt’s gods, the Canaanite gods, or no deity at all. But if that person wanted to live in the Promised Land, there was to be only one God, YHWH, for Israel.
SUMMARY: The Westminister Confession is a great statement of faith for the definition and description of God. Each generation must affirm or deny their belief in the existence of God and their relationship to him.
For more perspectives:
https://www.quora.com/What-is-God-and-why-did-God-reveal-Himself-in-the-First-Commandment-as-thy-God