All the Firstborn Were Yahweh's Because He Had Delivered Them at the Passover (Numbers 8:17).
“ For all the first-born among the children of Israel are mine, both man and beast.”
This leads up to the central fact, that the firstborn among the children of Israel were Yahweh's, whether man or beast (Numbers 3:13; Numbers 8:17; Exodus 13:2; Exodus 13:12; Exodus 22:29-30; Exodus 34:19). This specific fact is emphasised and is central to the passage (see analysis above). It was therefore no light thing that they were being replaced by the Levites. Here in the Levites were the representatives of every Israelite family, replacing their sons as representatives. That was why the people owed such a debt to the Levites, and were never to forget them (Deuteronomy 12:19).
So the dedicated Levites represented their dedicated firstborn, as the Nazirites represented their dedication as a people. Each was to be seen as indicating their responsibility towards Yahweh.
The Levites Taken Instead of All the Firstborn (Numbers 8:18).
“ On the day that I smote all the first-born in the land of Egypt I sanctified them for myself, and I have taken the Levites instead of all the firstborn among the children of Israel.”
The firstborn were Yahweh's because He had spared them when He smote all the firstborn in the land of Egypt (Exodus 11-13). As a result He had sanctified them for Himself for they had been under His special protection. Theirs was to be the privilege of serving His Sanctuary. But now these firstborn sons were released from their solemn privilege because Yahweh had chosen to take the Levites in their place. This was no doubt partly because of the recognition from experience of the untrustworthiness of the firstborn at times of crisis, in contrast with the trustworthiness of the Levites. But it was still a significant exchange.