“flint knives.” Freshly knapped flint knives are sharp as scalpels and sterile. They are perfectly suited for circumcision.
“knives.” The Hebrew word in the text of Joshua 5:2 and 5:3 is chereb (#02719 חֶרֶב), which is used over 400 times in the Old Testament and almost universally means a “sword” but does not mean a “knife.” The English Bibles almost all read “knives,” because knives were used for circumcision and also a sword cannot be made out of flint: the flint rock is not suitable for making a long blade and the weapon would be far too brittle to be used in battle. There are Hebrew words for “knife,” but God does not use them here, instead the text uses the word for “sword,” as some commentators point out (cp. David Howard, The New American Commentary). Although some lexicons give “knife” as a definition of the Hebrew word chereb, that is questionable because Joshua 5:2 and 5:3 are the only times chereb is translated “knife” in the Old Testament.
There is no doubt that the text uses the word “sword” on purpose. In telling Joshua to make “swords” and circumcise the Israelite men who had been born in the wilderness and who had never been circumcised, God was graphically pointing out and symbolically saying that we must make war on certain things that are lacking in our life—the enemy inside—before engaging the enemy outside and around us. God had said that an uncircumcised man was not in God’s covenant (Gen. 17:9-14; Exod. 12:43-49; Lev. 12:3). The Bible never says why Moses did not make sure that the male babies of Israel were circumcised on the eighth day after they were born as the Law said (Lev. 12:3), but Moses did not even circumcise his own boys; his wife Zipporah had to do it (Exod. 4:24-26). In any case, the men who were born during the forty years of wilderness wanderings were not circumcised, and so God let Joshua know that before he could fight the Canaanite enemy he had to deal with “the enemy in the camp,” which was Israel’s situation of not being in the Abrahamic Covenant. It is common to hear about the “wars of Joshua” and the 31 kings he defeated (Josh. 12:9-24) but it seems that from God’s perspective, the wars of Joshua included the “war” he fought among his own people to bring them into alignment with the Abrahamic Covenant.
God’s command to circumcise the men also took an act of trust on the part of Joshua and Israel. It takes some time to heal from circumcision, and it is very difficult to fight right after being circumcised. Jacob’s sons took advantage of that fact when they killed the men of Shechem (Gen. 34:13-25). From a military standpoint, it would have made a lot more sense to circumcise the men of Israel before they crossed the Jordan River than to cross the Jordan and then circumcise the men, because west of the Jordan they would be much more vulnerable to an attack from the Canaanites. By getting Israel to cross the Jordan and then circumcise the men, God was keeping up His demonstration that He was Israel’s key to victory, not their army or weapons.
“Yahweh said to Joshua.” Joshua 5:2-3 is another verse couplet that shows Joshua’s obedience to Yahweh (cp. Josh. 4:16-17).