The Old Testament is full of idol worship. It goes back to Genesis and seems to be a constant theme. Idolatry is not just worshiping statues. When Jacob decided it was finally time to leave his swindling father-in-law, Rachel, one of Jacob's wives, stole her father's idols. After the Israelites were delivered from Egypt they panicked because Moses had been gone up the mountain longer than they.
In this day and age, sex has become an idol that drives us to make small and big decisions that will lead us away from Christ if we're not careful. Money. Ok so it's not exactly a modern-day temptation only, but the lesson remains the same. There is much value in money, but it is not the most valuable thing. That's why Jesus teaches in Matthew.