Well most religions that use the Bible as their basic text BELIEVE that they believe the whole Bible. The reason they don't all agree is that there are many ways to interpret the Bible, or any other document of substantial size. The subject of interpretation is called "hermeneutics" -- the study of how we interpret documents. Seminaries and religious colleges teach classes in this, which go into great detail about the factors which influence interpretation of the Bible. The disturbing thing that comes up for me in this area is that almost everyone who posts Bible quotes on Answerbag does so from a mindset which seems to claim "and this is the ONLY right way to interpret this part of the Bible". That leads to a lot of conflicts.

From my experience as one of Jehovah’s witnesses, I can say that yes, we believe and use the entire Bible. We read, as a congregation, and at home, about four chapters of the Bible each week. Then at our school at our Kingdom Hall, we have two or even three talks on those chapters. We do this every week of the year. When we are finished, after about four years, we start it all over again. I have been through the Bible five times like this since my baptism. We believe in the words found at II Timothy Chapter 3, verses 16 and 17. There, Paul wrote the Christian overseer Timothy about the beneficialness of the Holy Scriptures: “All Scripture is inspired of God and beneficial for teaching, for reproving, for setting things straight, for disciplining in righteousness, that the man of God may be fully competent, completely equipped for every good work.” (2 Tim. 3:16, 17) As Paul said “all Scripture,” he meant the whole Bible, not just the so-called New Testament, which some religionists of Christendom say is all that is necessary for Christians, and not just the inspired Hebrew Scriptures, which the Jews say is all that is necessary for gaining a blessed life in the future world. We need the whole Bible, the pre-Christian part and the Christian part, for both parts are “inspired of God,” and we dare not ignore anything that God inspires. All of it is beneficial to seekers of everlasting life in happiness.

In addition to the excellent points raised by Stableboy, you also have to consider how people define the "whole Bible". For instance, Jews do not recognize the New Testament as part of the Bible. In addition, Christians recognize as Holy Scripture books which the Jews consider merely historical. So, from the Jewish perspective, they certainly believe the whole Bible.

jehovahs witnesses do. i am not one. have studied all main religions and they by far stick to the bible. As far as interpritations go, they have gone deeper than any other in finding the meanings behind each and every word and context, since there are so many dialects used througout the bible.