John Gill Commentary Numbers 15:26

"and all the congregation of the children of Israel shall be forgiven, and the stranger that sojourneth among them; for in respect of all the people it was done unwittingly." — Numbers 15:26 (ASV)

And it shall be forgiven all the congregation of the children
of Israel
Which is repeated for the certainty of it, and for the sake of what follows:

and the stranger that sojourneth among them ;
the proselyte of righteousness; so the blessing of pardon, through the propitiatory sacrifice of Christ, comes upon believing Gentiles as well as Jews, (Romans 4:9) (1 John 2:2) ;

seeing all the people [were] in ignorance ;
both the congregation of Israel and the stranger; (See Gill on Numbers 15:25).