Leviticus 18:30Therefore shall ye keep mine ordinance, that [ye] commit not [any one] of these abominable customs, which were committed before you, and that ye defile not yourselves therein: I [am] the LORD your God.
God says, "defile not yourselves," which means, don't make yourselves dirty. This what keeping the law ("ordinance,") was for. People have habits of doing very bad things God hates ("abominable customs".) Committing any 1 of those abominable customs would make you dirty. The Lord gave the Jews the old law to keep so they wouldn't do those things.
Today the Lord has given His Spirit to His believers. Faith (really thinking that Jesus is True,) takes action in us by Love. That means that we don't do anything that's bad for other people, but do all the good we can for everyone, especially for the brethren. This is the new law.
What abominable custom is the Lord speaking to you about at this moment? What habit are you doing that's so bad that the Lord and others hate it very much? Why make yourself filthy with a thing that the heathen do? Defile not yourselves.