‘ And Joshua said to the people, “You cannot serve YHWH, for he is a holy God, he is a jealous God, he will not forgive your transgressions nor your sins. If you forsake YHWH and serve foreign gods, then he will turn and do you hurt, and consume you, after having done you good.” '
But Joshua wanted no superficial reply. So he challenged them by pointing out the danger of making a covenant with YHWH. This was no God Who would stand by and do nothing. He was holy, set apart by the very nature of His being, unable and unwilling to put up with sin and disobedience. And He was a jealous God, unwilling to share worship with false gods who were no gods. Thus He would not overlook their sin and disobedience. If after swearing loyalty to Him they then pandered to foreign (having nothing to do with Israel) gods, He would bring evil on them and destroy them, even though He had previously done them good.
This was not, of course, a denial of the fact that He was a merciful God, but drew attention to the fact that sin without genuine repentance would reap its deserved reward. YHWH was not One Who could be mocked.
“You cannot serve YHWH.” This was a challenge to face up to their own weakness, revealed time and again in their past. It may contain within it the thought that they could not serve Him as He required because of the pagan influences they still allowed among them (Joshua 24:23). He wanted them to face up to the truth about themselves.
“He is a holy God.” The word for holy is in the plural, matching God (elohim). It is thus a plural of intensity. He is the sum of all that is holy. Isaiah 5:16 brings out something of its meaning. He is exalted as the great and righteous Judge and set apart by His total purity and goodness (compare Isaiah 57:15).
“He is a jealous God.” Not jealous in that He envies and feels sore about what others have and deserve, but aware of His own being and worthiness and unwilling to tolerate anything which puts on a pretence of sharing His uniqueness while being unable to do so. In other words he will not tolerate false gods. See Exodus 20:4; Exodus 34:14; Deuteronomy 4:24; Deuteronomy 6:15; Nahum 1:2. The use of El (singular) stresses the plural of intensity in the previous phrase. He is El-Qanno', the God of jealousy, the God so unique that He can have no rivals.