Why do Christians believe God always tells the truth?

Christians believe God always tells the truth; because they believe it is impossible for Him to lie. They believe the Bible is the truth because they believe the Bible is God's word.

 
Bible is like a plumb-line
 

The Bible is like a plumb-line to Christians. Christians believe the Bible is always right, just like a plumb-line. Like a plumb-line shows a builder a true and straight line, the Bible shows Christians how to live God’s way. Some people describe the Bible as ‘the Makers instructions’ – just as we might follow instructions on how to use a new piece of equipment the Bible describes how God planned His creation to be, it describes how He planned people to live. However the Bible is not just a list of instructions. Christians believe the Bible is a special work of the Holy Spirit.