Moses vs Joshua

[MountainChristian]

Moses

Deuteronomy 7:9-10 Know therefore that the LORD thy God, he is God, the faithful God, which keepeth covenant and mercy with them that love him and keep his commandments to a thousand generations; And repayeth them that hate him to their face, to destroy them: he will not be slack to him that hateth him, he will repay him to his face.

Joshua

Joshua 24:19 And Joshua said unto the people, Ye cannot serve the LORD: for he is an holy God; he is a jealous God; he will not forgive your transgressions nor your sins.

If Joshua was correct the LORD would have killed Joshua, all the Israelis, and every human on earth.  

[No Nicolaitans]

I think what Joshua was saying was...the people swore to serve God. With that said, he was warning them that if they then turned and rebelled against God, forsook him, and served false gods...after promising to serve God alone...God wouldn't forgive that.

Just like they did with Moses. They promised to serve God, and in no time, they forsook God, rebelled, and worshiped false Gods. Joshua saw it happen then, he saw the results, and he knew it would happen again. He then set a stone up as a witness against them so they would be reminded of their promise. 

In effect, they were promising to keep the law...which no man could or can do. Rather than making a rash promise, they should have cried out for grace. No man can serve God through the law, because the law shines the light and exposes our sinfulness. That (in my opinion) is why Joshua said they couldn't serve God. God is holy, and man is sinful.