“Then the Lord said to Moses, ‘When you take a census of the Israelites to count them, each one must pay the Lord a ransom for his life at the time he is counted. Then no plague will come on them when you number them. Each one who crosses over to those already counted is to give a half shekel… This half shekel is an offering to the Lord. All who cross over, those twenty years old or more, are to give an offering to the Lord'” (Exodus 30:13-14, NIV).
The census was actually a way to determine a record of military manpower. But, when Moses enlisted men for military service, he was also to take a ransom from each twenty-year-old or more male and use it for the construction and ministry of the Tabernacle.
“Crossing over to those already counted” literally meant passing over to those who are mustered. It meant joining the ranks of the enlisted men. By passing muster the Israelite male effectively became a soldier in the Israeli militia–the Lord’s army.