John Gill Commentary Joshua 24:22

"And Joshua said unto the people, Ye are witnesses against yourselves that ye have chosen you Jehovah, to serve him. And they said, We are witnesses." — Joshua 24:22 (ASV)

And Joshua said to the people
In reply to their answer and resolution:

you are witnesses against yourselves, that you have chosen the
Lord God to serve him ;
that is, should they, after this choice of him, which they had so publicly declared, desert his service, and go into idolatry, their testimony would rise up against them, and they would, be self-condemned:

and they said, we are witnesses ;
should we ever apostatize from the Lord and his worship, we are content to have this our witness produced against us.