At that time the LORD said to Joshua, “Make flint knives and circumcise the Israelites again.” So Joshua made flint knives and circumcised the Israelites at Gibeath Haaraloth (Josh 5:2-3).
This is one of the places in the Biblical narrative where geography plays an important role in understanding what is happening in this passage.
Joshua has taken over for Moses in leading Israel. To confirm this God parted the Jordan River at flood stage so that Israel could to cross into the land. This would imitate the great miracle He did in the leadership of Moses – the crossing of the Red Sea – and remind the people that Joshua was indeed God’s choice to succeed Moses. After the nation crossed, the river returned to its natural state.
The place that Israel crossed and camped was not far from the place where the Jordan River feeds into the Dead Sea. Geographically, this is the lowest point in elevation on the face of the earth. Within about 5 miles, and, more importantly, within sight was the fortified city of Jericho. Joshua, Israel’s military commander under Moses and now the political leader, was looking up at the city, with no place of escape behind him – not the place a military commander would seek to launch an attack from. It was at this point that God tells Joshua to circumcise his army, effectively disabling his army for 2-3 days. Had the king of Jericho tried, he could have launched an attack just then and destroyed completely the army that was threatening him. He, of course, didn’t know this but it didn’t make it any less significant that Joshua was risking the safety of his nation by immobilizing is army.
Why didn’t God have them do this before they crossed the Jordan? Why did He wait until the River had returned to flood stage? It was simply and solely because He wanted to test the faith of His leader. Joshua passed.
There are times when God tells His people to do what is totally against the dictates of human reason, but to do it at His command and in dependence upon Him. Tithing is such a command. In an age when there is such financial pressure on families, He still calls upon us to give a tithe (see Matt 23:23 and Luke 11:42). The idea is not that we deplete our resources; it that we honor the One who owns it all. And this often goes counter to accepted practice in our society.
A related area is that God promises us that if we seek first His kingdom, all our material needs will be cared for. So, should a Christian mom take a job and put her kids in day care or should she stay at home and instill the values in them that she believes? Should a teen take a part-time job that will require him/her to work on Sunday?
There are other apparently irrational things that God calls us to do that we should do in obedience, just like Joshua (e.g., consider Isaiah 40:31). If we are fully devoted to Him, He will test our faith.