Matthew 22:29 (NKJV)
There is an interesting story in Matthew 22 about a group of people who came to ask Jesus a question. These people, called the Sadducees, were part of a religious group that did not believe in the resurrection of the dead, but they were asking Jesus a question about what happens to people after the resurrection. What a strange topic for them to bring up. But, they did not really want to learn anything or have an actual answer to a real question; they were only trying to justify themselves and their beliefs while putting Jesus down.
These Sadducees were more well versed in their beliefs and opinions than what God had actually said. They did not really have an understanding of the Word of God. If they had been as informed about the entire Old Testament as they were about their own group's beliefs and opinions about the Old Testament they would not have said what they said. Jesus told them they were mistaken because they did not know the Scriptures. They knew part of the Scripture, the part that fit their beliefs, but they did not know the whole counsel of God, as Paul called it in Acts 20:27.
Just like the Sadducees, too many believers only know small portions of the Bible and those are usually the parts they like the best, they don't know the whole counsel of God. Too few Christians have a regular time of reading and studying the Word for themselves, they don't really know the Scriptures. We need to be people who know the Word of God for ourselves and are well acquainted with it from having spent time with it on a regular basis. We should all make it a habit to spent time with God through His Word, this will help us in our life and we will avoid making mistakes like the Sadducees did.