To the question, "what is God", most Christians have the same concept, but would probably express it differently. In fact, some may have great difficulty putting their answer to this question into words. What would be the proper response to the question, "what is God"?
First of all, God is a spirit. A spirit is an immortal being with an intellect and free will, but no body. God is unique among spirits in that He is the Supreme Being, meaning He is above all things. All creation is under His control. God is self-existing, meaning He owes His existence to nothing. He is the Creator, but was not created. God is also infinitely perfect, and because of this, He is immutable, meaning He has never changed, and will never change, because He cannot change (CCC 212).
What are some of God's perfections? For one thing, God is eternal. This means he always was and always will be. He has no beginning and will have no end. God is all-knowing, meaning He knows everything–what has happened, what is happening, and what will happen. God is all-present, meaning He is everywhere. He is also almighty, meaning He can do all things. Nothing is impossible for God (Luke 1:37). God is also all-good, all-wise, all-holy, all-merciful, and all-just, meaning His goodness, wisdom, holiness, mercy, and justice are perfect. God is also all-loving, His unconditional love being one of His great attributes (Is. 49:14-15; Jer. 31:3). Some say that a loving God would not permit the suffering that we see in the world. Although there is much evil and suffering in the world, it is not God who causes it or wills it, but He does allow it, because He respects the gift of free will that He gave to all men.
Some people are polytheistic, meaning they believe in many gods. But the Catholic Church, and the Jews before them, always taught that there is one and only one God (monotheism). At the time of Christ, however, it was revealed that although there is one God, with one divine nature, that one God consists of three persons–"the Father, the (Son), and the Holy Spirit" (1 John 5:8), the names by which we are baptized (Matt. 28:19). God the Father--the First Person of the Blessed Trinity, God the Son--the Second Person of the Blessed Trinity, and God the Holy Spirit--the Third Person of the Blessed Trinity, are equal. This means that all of God's perfections exist in all three Persons. That is why Jesus said, "I and the Father are one" (John 10:30). He was "equal with God" (John 5:!8).
Although the Father, the Son, and the Holy Spirit are equal, meaning they are all equally perfect, they have different roles. This is no different than men and women. Although they are equal, the husband is the head of the wife (1 Cor. 11:3).
God the Father is the Creator (Gen. 1-2), and is of no one, but it is from the Father that God the Son is begotten, and it is from the Father and the Son that the Holy Spirit proceeds.
God the Son, also known as the Word (John 1:1), became man while also remaining God, at the Incarnation when He was conceived by the power of the Holy Spirit (Luke 1:35). He was a divine person with both a divine and a human nature. Because of His divinity, Jesus retained all of His divine qualities, and therefore, His perfections. Because the divine will of Jesus Christ was in perfect harmony with His human will, He never sinned (2 Cor. 5:21), even though He was tempted by the devil (Matt. 4:1-11). In fact, because He was God, Jesus could not sin.
It is through God the Holy Spirit that God bestows His graces and gifts upon believers, especially through the sacraments of baptism and confirmation . The Holy Spirit moves us along the path of holiness. His gifts (Is. 11:2), charisms (1 Cor. 12:8-10), and fruits (Gal. 5:22-23) are all made manifest by His life in us.
But one may ask, how can Three Persons be one and the same God? They are one and the same God because they have the exact same nature. No Christian should be discouraged if he cannot fully comprehend the doctrine of the Trinity, because it is a mystery, which cannot be fully understood (CCC 234).