I note on almost all Bible apologetic website that the Battle of Jericho was justified because its residents were practicing abominable acts. But I've spent hours researching the Bible, and I can't find a specific passage that outlines the crimes being committed in Jericho. Please help me, for I'm in a debate with one of my atheistic friends over this issue.
God told Abraham that the Amorite were a wicked people.
"But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full." (Gen. 15:16, KJV)
Before crossing the Jordan, God told Joshua and the people that He was giving the land of Canaan to them because of the promise He made to Abraham, but also because the people were wicked.
"3 Understand therefore this day, that the Lord thy God is he which goeth over before thee; as a consuming fire he shall destroy them, and he shall bring them down before thy face: so shalt thou drive them out, and destroy them quickly, as the Lord hath said unto thee.
4 Speak not thou in thine heart, after that the Lord thy God hath cast them out from before thee, saying, For my righteousness the Lord hath brought me in to possess this land: but for the wickedness of these nations the Lord doth drive them out from before thee.
5 Not for thy righteousness, or for the uprightness of thine heart, dost thou go to possess their land: but for the wickedness of these nations the Lord thy God doth drive them out from before thee, and that he may perform the word which the Lord sware unto thy fathers, Abraham, Isaac, and Jacob." (Deu. 9:3-5, KJV)
Excerpt from The Peoples of Canaan:
"Numbers 13:29 indicates that "The Amalekites live in the Negev; the Hittites, Jebusites and Amorites live in the hill country; and the Canaanites live near the sea and along the Jordan." But the hills mentioned refer to the mountains east of the Jordan. The trouncing of the disobedient Israelites by the Amorites is retold in Deuteronomy 1:44.
The Amorites also lived west of the Jordan. Ai, conquered after Jericho, was an Amorite city (Joshua 7:7). Other Amorite populations attacked Israel's vassal, Gibeon." (1)
As Ai was an Amorite city west of the Jordan, Jericho is also presumed to have been an Amorite city.
The land of Canaan and all of the tribes of people who occupied it are many times referred to collectively as Canaanites. But, both Deuteronomy and Joshua refer to specific kings and cities by tribal names, and the tribes have overlapping boundary lines.
Their names are also confusing at times because some tribes called other tribes by different names. For example, the Rephaim were of the Amorite, and were called Zamzummim by the Ammonite, but also called Emim by the Moabite. (2)
We have to cross reference the names mentioned by God in order to find all of the details He gave us on the people of Canaan, but enough is there for us to know that He was fulfilling His promise to Abraham at the same that he used the Israelite invasion as a judgment against the wicked people of those territories.
God listed the evil things of which all the people of Canaan were guilty and commanded the Israelite that they were not to do in Lev. 18:3-29.
"3 After the doings of the land of Egypt, wherein ye dwelt, shall ye not do: and after the doings of the land of Canaan, whither I bring you, shall ye not do: neither shall ye walk in their ordinances." (Lev. 18:3, KJV)
Then in verses 4 - 24 God listed the things that the people of Canaan were doing: seeing the nakedness of family members, presumably for sexual relations; fornication; adultery; child sacrifice; homosexuality; and bestiality.
"24 Defile not ye yourselves in any of these things: for in all these the nations are defiled which I cast out before you:
25 And the land is defiled: therefore I do visit the iniquity thereof upon it, and the land itself vomiteth out her inhabitants." (Lev. 18:24-25, KJV)
God had told the Israelite to clear out all of the people of these lands because of these sins (Deu. 20:17-18). As Jericho was a city in that area, then Jericho was also practicing all of the sins listed in Lev. 18.
1) The Peoples of Canaan here
2) See "Giants: Rephaim, Zamzummim, Emim, Amorite, Anakim, Nephilim, Zuzim" at my blog ShreddingTheVeil