Nehemiah 10:28 And the rest of the people, the priests, the Levites, the porters, the singers, the Nethinims, and all they that had separated themselves from the people of the lands unto the law of God, their wives, their sons, and their daughters, everyone having knowledge, and having understanding.
Many of the returning Israelites had intermarried with the people of the land. Physical union mixed the purity of the Jewish people, but more importantly, the mixing of faith with false religious worship of demons diluted and polluted the spiritual union with a holy God. The influence of pagan wives would destroy the faith and devotion of the true believer in Yahweh. Nothing could be done to maintain the purity of faith except to separate the pagan wives from their midst. This drastic action of separation was undertaken under Nehemiah. This action helped to preserve faith in the one true God.
The issue for us NT Christians is to realize the necessity of separation from the world. It is not easy to be in the world but not of the world. We desire to be separated from the world around us so that worldly influences don’t seep away at our faith in Jesus for salvation, but it is hard to make those decisions in the real world. The problem is not solved by becoming Amish or something, but the issue of separation must be dealt with in the heart, internally not externally.
It is so complicated that human reason cannot discern and human effort cannot do. We desperately need to hear the Gospel regularly so that the Holy Spirit can work in our hearts “to will and to do,” and “to bring every thought captive to Christ.” The Spirit is our Helper in this regard, and it is He who separates the soul with its worldly desires from the spirit with its godly desires.