THE MEANING OF GOD'S ONENESS
It is obvious that God's oneness differs from man's oneness. Man's oneness makes him limited. Thus it is not possible for one man to be in two places at the same time. But God can be on His throne in heaven and, at the same time, be here on this earth. This is neither impossible nor difficult for God. And we do not mean that part of Him is in heaven while the other part is on the earth. We mean that God, in all His glory, can be on His throne in heaven and at the same time be here on this earth.
This is exactly what happened when Christ came to this earth. The Bible states clearly that in Christ "dwells all the fullness of the Godhead bodily" (Colossians 2:9). This is what we call the Incarnation. We have heard of great men and kings who wore poor people's clothes and went to visit the poor in their homes without taking servants with them in order not to frighten the poor, and in order to see their needs and to help them. We admire such great men, and praise them for their humility and nobility. The greater the person, the greater is his nobility in humbling himself.
But who is the greatest of all? Is it not God Almighty? And who is the most noble? Is it not He who created them all? To sum up then, we assert, and God unequivocally declares, that "God is one!" But His oneness is not the same as man's oneness, because God is not limited. And the human mind cannot fathom the depth of what deity is. It therefore suits man to stand in awe when he considers what God revealed about Himself.
WHAT GOD REVEALED
Man cannot truly know about God except through divine revelation. It is not possible for the finite human mind to understand the nature of the infinite God. Therefore, it was necessary that God reveal Himself to us through divinely inspired writings, and that He protect and guard them from any attempt to change even one word or one letter. It is certain that God's Word abides forever, as the prophet had said long ago: "Forever, O Lord, your word is settled in heaven" (Psalm 119:89). And the New Testament declares that "The word of God ... lives and abides forever" (1 Peter 1:23).
THE NATURE OF THE GODHEAD
Before Christ ascended to heaven, He commanded His disciples saying, "All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit" (Matthew 28:18,19). Notice that He did not say to them, "in the names of the Father and the Son, and of the Holy Spirit." He said, "in the name" because it is one name—the glorious name of the one and only true and living God.
But someone might object and say, "How can that be? How can three be one?" This objection rises from the great error in trying to apply the rules of physics and mathematics to the Godhead. Herein lies the mistake and the cause of the perplexity. The Almighty God, who created nature and imposed on it rules of mathematics and other natural laws, is Himself not under such laws. We cannot, and should not, apply the laws of created material to spiritual issues.
FATHER, SON, AND HOLY SPIRIT: ONE GOD
Clearly then, the Holy Bible teaches us that there is only one God. Now let's consider how that agrees with the mention of the Father, the Son, and the Holy Spirit in seven places in the Bible.
"Baptizing them in the name (not names) of the Father, and of the Son, and of the Holy Spirit" (Matthew 28:19). It is one name of one God as we have seen.
Of true believers, who have received Christ in their hearts and thus were born again by a new spiritual birth. Christ said: "My sheep hear my voice, and I know them, and they follow me, and I give them eternal life; and they shall never perish, neither shall anyone snatch them out of My hand. My Father, who has given them to me is greater than all, and no man is able to snatch them out of my Father's hand. I and My Father are one" (John 10:27-30).
"Philip said to Him, ‘Lord, show us the Father, and it is sufficient for us.’ Jesus said to him, ‘Have I been with you so long and yet you have not known Me, Philip? ... He who has seen Me has seen the Father ... I am in the Father, and the Father in Me ... the Father who dwells in Me does the works’" (John 14:8-10).
"But you are not in the flesh but in the Spirit, if indeed the Spirit of Christ dwells in you. Now if anyone does not have the Spirit of Christ, he is not His"—meaning, he does not belong to Christ (Romans 8:9). Thus the Holy Spirit of God is also called the Spirit of Christ because of the oneness of God.
"But as it is written: ‘Eye has not seen, nor ear heard, Nor have entered into the heat of man the things which God has prepared for those who love Him.’ But God has revealed them to us through His Spirit ... which things we also speak ... which the Holy Spirit teaches" (1 Corinthians 2:9-13). Thus the Holy Spirit is spoken of as the Spirit of God repeatedly through the Word of God.
"But Peter said, ‘Ananias, why has Satan filled your heart to lie to the Holy Spirit? ... you have not lied unto men but unto God’" (Acts 5:3,4). Thus we see that lying to the Holy Spirit is lying to God because the Holy Spirit is God and is one with the Father and Son.
Jesus said, "But if I cast out demons by the Spirit of God, surely the kingdom of God has come upon you" (Matthew 12:28). Thus we see that the Son is in the Father, and the Father is in the Son. The Holy Spirit is the Spirit of God and He is the Spirit of Christ. There are also verses that begin with the statement, "Thus says the Lord" which are quoted elsewhere with "Thus says the Holy Spirit."
We see therefore that the Bible teaches us that God is one, and that the Father, the Son, and the Holy Spirit are the one and only true and living God. The difficulty is in the limitation of the human mind, and the wrong attempt to analyze God in the same way that we analyze matter—according to the laws of physics, chemistry and other sciences. These laws apply to matter, but should not and cannot be applied to God.
Taken from: http://www.answering-islam.org/god/only_true_god.html
It is obvious that God's oneness differs from man's oneness. Man's oneness makes him limited. Thus it is not possible for one man to be in two places at the same time. But God can be on His throne in heaven and, at the same time, be here on this earth. This is neither impossible nor difficult for God. And we do not mean that part of Him is in heaven while the other part is on the earth. We mean that God, in all His glory, can be on His throne in heaven and at the same time be here on this earth.
This is exactly what happened when Christ came to this earth. The Bible states clearly that in Christ "dwells all the fullness of the Godhead bodily" (Colossians 2:9). This is what we call the Incarnation. We have heard of great men and kings who wore poor people's clothes and went to visit the poor in their homes without taking servants with them in order not to frighten the poor, and in order to see their needs and to help them. We admire such great men, and praise them for their humility and nobility. The greater the person, the greater is his nobility in humbling himself.
But who is the greatest of all? Is it not God Almighty? And who is the most noble? Is it not He who created them all? To sum up then, we assert, and God unequivocally declares, that "God is one!" But His oneness is not the same as man's oneness, because God is not limited. And the human mind cannot fathom the depth of what deity is. It therefore suits man to stand in awe when he considers what God revealed about Himself.
WHAT GOD REVEALED
Man cannot truly know about God except through divine revelation. It is not possible for the finite human mind to understand the nature of the infinite God. Therefore, it was necessary that God reveal Himself to us through divinely inspired writings, and that He protect and guard them from any attempt to change even one word or one letter. It is certain that God's Word abides forever, as the prophet had said long ago: "Forever, O Lord, your word is settled in heaven" (Psalm 119:89). And the New Testament declares that "The word of God ... lives and abides forever" (1 Peter 1:23).
THE NATURE OF THE GODHEAD
Before Christ ascended to heaven, He commanded His disciples saying, "All authority has been given to Me in heaven and on earth. Go therefore and make disciples of all the nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit" (Matthew 28:18,19). Notice that He did not say to them, "in the names of the Father and the Son, and of the Holy Spirit." He said, "in the name" because it is one name—the glorious name of the one and only true and living God.
But someone might object and say, "How can that be? How can three be one?" This objection rises from the great error in trying to apply the rules of physics and mathematics to the Godhead. Herein lies the mistake and the cause of the perplexity. The Almighty God, who created nature and imposed on it rules of mathematics and other natural laws, is Himself not under such laws. We cannot, and should not, apply the laws of created material to spiritual issues.
FATHER, SON, AND HOLY SPIRIT: ONE GOD
Clearly then, the Holy Bible teaches us that there is only one God. Now let's consider how that agrees with the mention of the Father, the Son, and the Holy Spirit in seven places in the Bible.
"Baptizing them in the name (not names) of the Father, and of the Son, and of the Holy Spirit" (Matthew 28:19). It is one name of one God as we have seen.
Of true believers, who have received Christ in their hearts and thus were born again by a new spiritual birth. Christ said: "My sheep hear my voice, and I know them, and they follow me, and I give them eternal life; and they shall never perish, neither shall anyone snatch them out of My hand. My Father, who has given them to me is greater than all, and no man is able to snatch them out of my Father's hand. I and My Father are one" (John 10:27-30).
"Philip said to Him, ‘Lord, show us the Father, and it is sufficient for us.’ Jesus said to him, ‘Have I been with you so long and yet you have not known Me, Philip? ... He who has seen Me has seen the Father ... I am in the Father, and the Father in Me ... the Father who dwells in Me does the works’" (John 14:8-10).
"But you are not in the flesh but in the Spirit, if indeed the Spirit of Christ dwells in you. Now if anyone does not have the Spirit of Christ, he is not His"—meaning, he does not belong to Christ (Romans 8:9). Thus the Holy Spirit of God is also called the Spirit of Christ because of the oneness of God.
"But as it is written: ‘Eye has not seen, nor ear heard, Nor have entered into the heat of man the things which God has prepared for those who love Him.’ But God has revealed them to us through His Spirit ... which things we also speak ... which the Holy Spirit teaches" (1 Corinthians 2:9-13). Thus the Holy Spirit is spoken of as the Spirit of God repeatedly through the Word of God.
"But Peter said, ‘Ananias, why has Satan filled your heart to lie to the Holy Spirit? ... you have not lied unto men but unto God’" (Acts 5:3,4). Thus we see that lying to the Holy Spirit is lying to God because the Holy Spirit is God and is one with the Father and Son.
Jesus said, "But if I cast out demons by the Spirit of God, surely the kingdom of God has come upon you" (Matthew 12:28). Thus we see that the Son is in the Father, and the Father is in the Son. The Holy Spirit is the Spirit of God and He is the Spirit of Christ. There are also verses that begin with the statement, "Thus says the Lord" which are quoted elsewhere with "Thus says the Holy Spirit."
We see therefore that the Bible teaches us that God is one, and that the Father, the Son, and the Holy Spirit are the one and only true and living God. The difficulty is in the limitation of the human mind, and the wrong attempt to analyze God in the same way that we analyze matter—according to the laws of physics, chemistry and other sciences. These laws apply to matter, but should not and cannot be applied to God.
Taken from: http://www.answering-islam.org/god/only_true_god.html