John Calvin Commentary Numbers 15:14

"And if a stranger sojourn with you, or whosoever may be among you throughout your generations, and will offer an offering made by fire, of a sweet savor unto Jehovah; as ye do, so he shall do." — Numbers 15:14 (ASV)

And if a stranger sojourn with you. He does not mean all strangers, but only those who, descending from Gentile nations, had professedly turned to God, and thus had been received into the body of the Church. For the uncleanness of those who remained in uncircumcision excluded them from the legal service.

I conceive that there were two reasons why God would have one and the same form observed:

1. First, that the proselytes who had been recently incorporated might more cheerfully devote themselves to the exercises of piety, when they saw themselves placed in the same position as the children of Abraham.
2. And secondly, lest if any distinction should be made, corrupt mixtures should immediately creep in.

Lest, therefore, the purity of God’s worship should be gradually corrupted by absurd imitation, the gate was shut against that variety which usually draws people aside in different directions.