The Levites could only undertake their duties (Numbers 3; Numbers 4) after the formal exchange of the Levites for the first-…

Instead of such as open every womb, even instead of the firstborn of all the children of Israel, have I taken them unto me.

For…

And I have taken the Levites for all the firstborn of the
children of Israel .
] Gave…

Here we have directions for the solemn ordination of the Levites. All Israel must know that they did not take this honor to themselves, but were ca…