It most certainly is. It is not enough to believe in God. It is imperative to KNOW God. To know who He is, to know what He says is right, to know what He says is wrong. This is why He revealed Himself through the Bible and in Christ.
Many people believe in God, but they don't believe in THE God. Just as God created man in his image, many people create God in their own image. They invent a God that agrees with them, rather than following the commandments of the one true God. They worship a God who doesn't exist, or demons masquerading as God. This is why Christ said that not everyone who says "Lord, Lord" will inherit the Kingdom. It is not enough to believe in a divine being. One must recognize the identity of who that specific divine being actually is. And the only way to recognize that is to follow His commandments, not the commandments of men or false gods.