The statement was made that Christians and Muslims and Jews all claim to be monotheistic and all claim to worship God (or in the case of Muslims, Allah), therefore they all worship the same God.
The problem with this is that the person who made the statement is not considering the facts. For instance, two people could make two different claims and neither claim be true. Alternatively, one claim could be true and the other not true. Finally, both claims could be true. To make a determination as to which is the case, one must examine the evidence of the claims.
Judaism, Christianity, and Islam each believes there is one god who created the universe. They have many similarities in their moral codes, although Islam and Judaism are closer in that regard than Christianity – both practice not eating pork, circumcision, daily prayer and fasting during the first month of the year. All three religions honor Abraham. In fact, each claims to worship the god worshipped by Abraham. But the similarity between Islam and the other two ends there . Islam claims the son Abraham was told to sacrifice was Ishmael, despite the fact that Genesis 22 explicitly states it was Isaac. Judaism and Christianity believe God and hold that it was Isaac (http://en.wikipedia.org/wiki/Prophet_Ibrahim).
As the web site Judaism Online (http://www.simpletoremember.com/vitals/IslamJudaism.htm) states, “The real difference between the two religions, however, lies in their basis for belief. Judaism is based on the unique historical event of a divine revelation experienced by the entire nation. Whereas Islam is based on the prophetic claims of a single individual who subsequently convinced others to follow his ways.”
Neither Islam nor Judaism believe God is a Trinity, but Christians in the realm of orthodoxy believe that God is three in one – one God in three Persons. Christianity holds that there is God the Father, God the Son, and God the Holy Spirit. This idea is based on these Bible verses: Jn 6:27, Jn 1:1-2, Mat 28:19, Jn 14:16-17, Jn 15:26, Jn 17:5, Heb 9:14, Jn 10:30, Isa 45:5-6 (http://wbmoore.wordpress.com/2008/12/21/what-is-the-doctrine-of-the-trinity/).
The True God is not the Islamic Allah. Just because religions claim to be monotheistic does not mean what they think is god is in fact God. It may mean, as it does in this case, that someone is wrong.
Not all religions are the same, nor do they worship the same being.
Judaism claims the same god as Christianity, even though Jews do not believe in the trinity, and in fact deny Jesus is the Messiah.
29 Is God the God of Jews only? Is he not the God of Gentiles too? Yes, of Gentiles too, 30 since there is only one God, who will justify the circumcised by faith and the uncircumcised through that same faith.
There IS only one God. But while Scripture is clear that they must have faith, what they have faith in is important.
There is only one way to be with God, and that is to trust in God – through faith in Jesus Christ who was sent by God the Father to suffer and die for our sins.
In fact, Scripture is clear that to deny Jesus is the Messiah is to lie, to be anti-christ, to deny the Father and the Son.
1 John 2:22-23
22 Who is the liar? It is the man who denies that Jesus is the Christ. Such a man is the antichrist – he denies the Father and the Son. 23 No one who denies the Son has the Father; whoever acknowledges the Son has the Father also.
1 John 5:20
We know also that the Son of God has come and has given us understanding, so that we may know him who is true. And we are in him who is true – even in his Son Jesus Christ. He is the true God and eternal life.
To deny Christ is to deny God, thus Islam (which denies Jesus is Messiah) does not worship the True God. Judaism, though they might believe they worship God, they deny His revelation and gift of eternal life through having sent His Son to suffer and die for our sins. In doing so, they show a lack of faith in God and so deny God.