Israel of old was a chosen nation. A royal people made Holy by the GOD who pursued her heart. GOD set them up in a land of enemies, and told them that they were to be priests, an example to those around them to the power of this LIVING GOD!
Jesus calls us out today in much the same way. We are to be salt and light. Two things very common today, so common that I forget the necessity that both were in the 1st Century.
You see, Satan knows God, and he knows us as well. Our enemy realizes that we can be distracted from our purpose and our focus on YHWH. The lesson that we today can carry from Israel of the Old Testament is this: As long as Israel served the LORD their GOD, they were a mighty force and their enemies feared them. When they served themselves or gods of the land...they were defeated by the surrounding nations (read the OT book of Habakkuk).
Jesus says that although we are salt, if we lose our saltiness (JESUS) then we are only good for the trampling of the feet of our enemy. When our hearts turn from GOD and from serving HIM we are not worthy of the dung heap and the world will see us fall because we are a city on a hill.
To quote the Apostle Peter, we must be "evermore eager to make our calling and election secure."