“ And if it seem evil to you to serve YHWH, choose you this day whom you will serve, whether the gods your fathers served who were beyond the River, or the gods of the Amorites in whose land you dwell. But as for me and my house, we will serve YHWH.”
Now that it was the future that was being challenged the gods of Canaan were introduced. Joshua challenged them as to whether they would serve their ancestors' gods, or the gods of the Amorites (the Canaanites under another name), who had done nothing for them, or YHWH, Who had done so much for them. We can compare 1 Kings 18:21 for a similar challenge. It was an important challenge, and was no light choice. It was choosing between the God Who made righteous demands and expected a strict morality, and gods who made no moral demands and would introduce them to sexual perversions and lascivious living.
“But as for me and my house, we will serve YHWH.” Joshua had no doubt as to where he stood and became the first to make his declaration as an example to the remainder.