God
The English word "God" comes from the Old English "god", which itself is derived from the Proto-Germanic "*gudą". This Proto-Germanic form dates back to ancient times, likely over 2000 years ago.  The ultimate origin may go back even further to the Proto-Indo-European root "*ǵhu-tó-m", which was likely based on the root "*ǵhau(ə)-", meaning either "to call" or "to invoke". This would place the very earliest origins of the word around 4000-5000 years ago.  The earliest written form of the Germanic word "God" comes from the 6th-century Christian Codex Argenteus.  Before monotheism, the word likely had a general, predominantly plural or collective sense referring to multiple deities.  ​It began to be used in the singular to refer specifically to the Christian deity after conversion to Christianity.
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"God is love, and those who live in love live in God, and God lives in them."

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​Baha'i Faith:  "God is the unknowable essence, exalted above everything that can either be recounted or perceived."     Bahá'u'lláh, Gleanings from the Writings of Bahá'u'lláh
"God is the creator of all worlds, visible and invisible, known and unknown."   Bahá'u'lláh, Prayers and Meditations
Buddhism:  "Infinite compassion, infinite wisdom, and infinite love; these are the divine qualities of the awakened mind." The Lotus Sutra
"There is no independent creator in Buddhism; however, divinity resides in the Buddha-nature within all sentient beings."   Mahayana Buddhist Texts
Christianity:  "God is love, and those who live in love live in God, and God lives in them."  1 John 4:16
"In the beginning was the Word, and the Word was with God, and the Word was God." John 1:1
Confucianism:  "Heaven is the will and the source of all things. Heaven and Earth together sustain the world."  Confucius, The Analects
"Heaven’s mandate is not a constant thing; when the right is pursued, the mandate is given."     The Doctrine of the Mean
Hinduism:  "God is one, but wise men call Him by different names."   Rig Veda 1.164.46
"Thou art the unborn, the beginningless, the Lord of all beings."     Bhagavad Gita 10:3
Islam:  "Say: He is Allah, the One; Allah, the eternal, the absolute."   Quran 112:1
"Allah is the Light of the heavens and the earth."     Quran 24:35
Jainism:  "The pure soul, which has neither beginning nor end, is God."   Tattvartha Sutra
"Infinite knowledge, infinite perception, infinite bliss, and infinite power—these are the attributes of the true divine."   Acaranga Sutra
Judaism:  "Hear, O Israel: The Lord our God, the Lord is one."   Deuteronomy 6:4
"I am that I am."   Exodus 3:14
Shinto:  "The kami (gods) are the divine spirits that inhabit all things, seen and unseen, and govern the forces of nature."   Shinto Teachings
"The gods manifest in the world of nature, from the mountains to the seas, and protect humanity."   Shinto Wisdom
Sikhism:  "God is one, the eternal truth, the creator of all, without fear, without enmity, timeless and self-existent."   Guru Granth Sahib, Japji Sahib
"There is but one God. He is the Creator, Sustainer, and Destroyer of the universe."   Guru Granth Sahib, 1:1
Stoicism (Philosophy):  "God is the universal reason, the mind that pervades all things." Marcus Aurelius, Meditations
"God is nature; what happens is according to His will."    Epictetus, Discourses
Taoism:  "The Tao that can be spoken of is not the eternal Tao. The Tao is the origin of Heaven and Earth."   Tao Te Ching, Chapter 1
"Tao is the great mother: empty yet inexhaustible, it gives birth to infinite worlds."  Tao Te Ching, Chapter 6
Zoroastrianism:  "Ahura Mazda is the supreme creator, the source of all light and wisdom."     Avesta, Yasna 31.7
"Through the divine will of Ahura Mazda, all good deeds, thoughts, and words come into being."
  Avesta, Gathas
Ancient Greek Philosophy (Plato):  "God is the creator of the cosmos, the divine craftsman who shapes the universe with order and purpose."   Plato, Timaeus
"God is the ultimate good, the source of all truth and beauty."   Plato, Republic
Epicureanism (Philosophy):  "The gods exist, but they are indifferent to human affairs, living in perfect peace and happiness."   Epicurus, Letter to Menoeceus
"The gods are not to be feared; they have no interest in our lives."  Epicurus, Principal Doctrines
Humanism (Philosophy):  "God is a concept by which we measure our pain." John Lennon, God
"God is the unity of moral and natural law, perceived in human reason."  Immanuel Kant, Critique of Pure Reason
New Age Philosophy:  "God is the infinite energy that connects all things, both seen and unseen."   The Celestine Prophecy
"The divine spark exists within every individual, linking all to the universal consciousness."
  New Age Teachings
Native American Spirituality:  "The Great Spirit is in all things: he is in the air we breathe, he is in the trees and rocks, he is in the buffalo and the streams."   Black Elk, Lakota Wisdom
"All things in creation are connected by the Great Spirit."     Ojibwa Teachings
Unitarian Universalism: 
"God is love, and love is the spirit of this church."     UU Principles
"God is that force which draws us to seek the truth, to do good, and to love others."   UU Teachings​

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Ancient Greek Philosophy: "The unmoved mover" (Aristotle)
​Baha'i Faith: "There is one God, known by many names" (Baha'i teachings)
Buddhism: "Buddha-nature" (Mahayana Buddhism concept)
Christianity: "God is love" (1 John 4:8)
"I am who I am" (Exodus 3:14)
"Our Father who art in heaven" (Matthew 6:9)
"The Alpha and the Omega" (Revelation 1:8)
Confucianism: "Heaven" (Tian in Confucian texts)
Daoism: "The Dao that can be spoken is not the eternal Dao" (Tao Te Ching)
Deism: "The divine watchmaker" (William Paley)
Gnosticism: "The true God beyond the Demiurge" (Gnostic teachings)
Hinduism: "Brahman" (Upanishads)
"Tat Tvam Asi" (That Thou Art) (Chandogya Upanishad)
"The Atman is Brahman" (Advaita Vedanta philosophy)
Islam: "Allah" (The Quran)
"There is no god but God" (Shahada)
Jainism: "The perfected soul" (Paramatman in Jain philosophy)
Judaism: "Adonai" (Hebrew Bible)
"Ein Sof" (Kabbalah)
"Elohim" (Hebrew Bible)
"YHWH" (Tetragrammaton in Hebrew Bible)
Native American Spirituality: "The Great Spirit" (Various Native American traditions)
Neoplatonism: "The One" (Plotinus)
New Age: "The Universe" (Various New Age teachings)
Pantheism: "God is everything and everything is God" (Spinoza)
Panentheism: "The world is in God" (Various panentheistic philosophies)
Philosophy: "The Absolute" (Hegel)
"The First Cause" (Thomas Aquinas)
"The Ground of Being" (Paul Tillich)
Rastafarianism: "Jah" (Rastafarian teachings)
Shinto: "Kami" (Shinto beliefs)
Sikhism: "Ik Onkar" (Sikh scriptures)
Stoicism: "The Logos" (Stoic philosophy)
Sufism: "The Beloved" (Sufi poetry and teachings)
Taoism: "The Way" (Tao Te Ching)
Theravada Buddhism: "Nirvana" (Pali Canon)
Unitarian Universalism: "The divine spark within all" (Unitarian Universalist principles)
Vaishnavism: "Krishna" (Bhagavad Gita)
Zoroastrianism: "Ahura Mazda" (Avesta)
Zurvanism: "Zurvan" (Ancient Persian religion)