Flint knives ( Joshua 5:2) were sharp flint rocks (obsidian). The first mass circumcision of the Israelites evidently took place in Egypt before the first Passover and the Exodus. [source][source][source]
"The sentence upon the fathers, that their bodies should fall in the desert, was unquestionably a rejection of them on the part of God, an abrogation of the covenant with them. This punishment was also to be borne by their sons; and hence the reason why those who were born in the desert by the way were not circumcised." [1][source]
Another explanation is that most of the older generation simply neglected to circumcise their sons out of forgetfulness, discouragement, or for some other reason. [source][source][source]
Why did God wait to command the circumcision of the new generation until now rather than on the plains of Moab? Perhaps He did so because He wanted to bring the people into the land before enforcing this aspect of the Law. This is consistent with God"s dealings with humankind. He first gives and then asks (cf. Romans 12:1). [source][source][source]
"Had Joshua acted on the principles common to all other generals, when invading an enemy"s country, he would either have prosecuted his advantages instantly, while his enemies were filled with terror, and crushed them before they had time to prepare for their defence [2]; or he would have fortified his own camp to prevent surprise, and to be in constant readiness for any emergency that might arise. But instead of adopting any military plans whatever, the very day after he had invaded the country, without waiting to know what effect the invasion would have, he appoints nearly every male in the congregation to be circumcised! Thus by one act disabling the greater part of his whole army from even standing in their own defence [2]! What but a principle of the most triumphant faith could have brought them to submit to such an injunction as this?" [4][source]
The reproach of Egypt ( Joshua 5:9) was the charge that originated with the Egyptians that Yahweh had led the Israelites out of Egypt only to destroy them in the wilderness (cf. Exodus 32:12; Numbers 14:13-16; Deuteronomy 9:28). Now that He had brought them into the land He had promised them, He had negated or "rolled away" this criticism. Gilgal sounds like the Hebrew word translated "rolling" (galal) and means "circle." Another view is that the reproach of Egypt refers to the disgrace the Israelites experienced in Egyptian slavery. When the Israelites obeyed God by circumcising their young men, the Lord"s deliverance of them reached its climax. [5][source]
The Israelites seem to have regarded the rolling away of the foreskins in the circumcision operation as having a double symbolic meaning. It represented God"s removal of their reproach as well as their renunciation of the flesh (cf. Genesis 17). [source][source][source]
"Flint knives [6] are sharpened by chipping away at the edge of the stone, so that clean, sterile stone is exposed, since bacteria and viruses cannot grow in rock. Circumcision was thus performed with an instrument possessing comparable sterility to today"s surgical scalpels. In view of the likelihood of infection following this operation with a contaminated instrument, use of the flint knife was enormously beneficial and therefore commanded by the Ultimate Healer (or in this case the preventer). [7][source]
God specified knives of flint even though this was the Late Bronze Age (1550-1200 B.C.), and bronze implements were common. [source][source][source]