Stones in general represent true ideas, according to Swedenborg. Flint is a kind of stone that can be chipped to form extremely sharp edges. To keep the metaphor going we would want flint to be truth that “comes to a point” – and in a way that’s exactly what it is.
According to Swedenborg, flint represents the “truths of faith” – the rules we learn about how to be good people and serve others so that we can apply them as situations arise in life. These are relatively black-and-white concepts (“Thou shalt not murder”) that help us identify evil desires and arm ourselves against them.
This is why flint knives were (and in some cases still are) used for circumcision, which represents purification from our most base love of self and love of the world – something that must be accomplished by truths of faith.
Passages from Swedenborg
Arcana Coelestia (Elliott) n. 2046
It is by means of cognitions of truth therefore that a person is reformed and regenerated, but this does not take place before he has been endowed with them. Conscience itself is formed by means of the truths of faith, for the conscience with which a regenerate person is endowed is a conscience for what is true and right. On this matter see 977, 986 (end), 1033, 1076, 1077. This also is the reason why flint knives, or ‘swords of flint’, as they are called, were used for circumcision. ‘Swords of flint’ means truths, see above 2039 (end).
Arcana Coelestia (Elliott) n. 2799
[6] But as for ‘small swords’ or ‘knives’ meaning the truth of faith, this becomes clear from the fact that they were used not only in sacrifices but also in circumcision. Those used in circumcision were made of flint and were called ‘small swords of flint’, as is evident in Joshua,
Jehovah said to Joshua, Make small swords of flint for yourself; and again, circumcise the children of Israel a second time. And Joshua made small swords of flint for himself, and circumcised the children of Israel on the hill of the foreskins. Josh. 5:2, 3.
‘Circumcision’ was a representative of purification from self-love and love of the world, see 2039, 2632. And because such purification is achieved by means of the truths of faith, ‘small swords of flint’ were therefore used, 2039 (end), 2046 (end).
Arcana Coelestia (Elliott) n. 7044
7044. ‘And Zipporah took a flint’ means that the representative Church used truth to demonstrate the actual nature. This is clear from the representation of ‘Zipporah’ here as the representative Church; and from the meaning of ‘a flint’ as the truth of faith. The use of knives made of flint to carry out circumcision meant the use of the truths of faith to carry out purification from filthy kinds of love, 2039, 2046, 2799; for circumcision was an act that represented purification from those kinds of love, 2799. The reason why purification is effected by means of the truths of faith is that these truths teach what good is and also what evil is, and so what ought to be done and what ought not to be done. And when a person knows those truths and wishes to act in accordance with them, he is being led by the Lord and purified by the Divine means that are His. Since the truths of faith teach what evil is and what good is it is evident that ‘Zipporah took a flint’ means the use of truth to demonstrate the actual nature.
Arcana Coelestia (Elliott) n. 7918
For the earthly and worldly types of love from which a person has to be purified are not recognized except by means of truths. When these truths are instilled by the Lord a horror of those types of love, as of things unclean and damnable, is also at the same time instilled. The effect this horror has is that when something similar enters the person’s thinking the feeling of horror returns, producing a loathing of those types of love. That is how a person is purified by truths, which serve as outward means. This was the reason why it was laid down that knives or ‘small swords’ made of flint should be used when circumcision took place, ‘small swords’ or knives made of flint being the truths of faith by means of which purification is effected, see 2799, 7044, and ‘circumcision purification from filthy types of love, 2039, 2632, 3412, 3413, 4461, 7045.
AE 405 (Tansley)
“He turned the rock into a pool of waters, and the flint into a fountain of waters.” By a pool of waters are signified knowledges of truth, and by a fountain of waters is signified the Word from which these are; and by the rock, the natural man as to truth before reformation, and by the flint, the natural man as to good before reformation.