why is the Law described as a burden in the Bible?
The Law is called a burden because it rested heavily on those under it, weighing them down without liberty and with constant fear. Violations, such as picking up a stick on the Sabbath, brought death. Israel remained under this burden continually until the faith that should afterward be revealed.
Les explains that the Law was a burden in the same way a beast of burden is loaded down: it weighed heavily on the people under it. They knew nothing of liberty and lived in constant fear of it, because even picking up a stick on the Sabbath brought death, not a slap on the wrist. Before the Faith Way, Israel was under the burden of the Law constantly, 24 hours a day, and was kept shut up under it until the faith which should afterward be revealed. This fits Scripture as progressive revelation: God did not tell everything up front, nor did He tell Israel and the world that Christ would one day die and send grace. Israel went 1,500 years under the Law before the doctrines of grace were progressively revealed.