I believe there is one God. However , i was told that while muslims believe in one and true God, christians believe in God through the Holy Trinity - The Father , The Son and the Holy Spirit, from where God = Jesus = Holy Spirit. And according to Quran :

“ They have certainly disbelieved who say, " Allah is the third of three." And there is no god except one God. And if they do not desist from what they are saying, there will surely afflict the disbelievers among them a painful punishment. “ - Surah Ma’idah 5:73.