Interest is not evil

You shall not charge interest to your brother—interest on money or food or anything that is lent out at interest. To a foreigner you may charge interest, but to your brother you shall not charge interest, that the Lord your God may bless you in all to which you set your hand in the land which you are entering to possess. - Deuteronomy 23:19-20 (NKJV)

There's a simple rule I like to apply when trying to figure out if something is "good" or "bad": did God ever tell his people to do it or not to do it? This won't always provide the whole picture, but serves as a great starting place for the investigation. If God did not provide any sort of direction or commentary, then sometimes looking at motivations or "heart condition" can still be worthwhile.

In this case, God provided direction to Israel that they should not charge one another interest, but if loaning money outside Israel, they could charge interest. This means that interest is not evil, because God being good is not going to provide a command to his people which is not also good.

I do often see a lot of excitement around usury, which is often understood as excessive interest, but lacking a good objective measure for what is "fair", it can be difficult to have a cogent conversation. Much like with the topic of circumcision, albeit in a financial setting, I don't really ever see anyone explain why God was wrong to command something for his people. Why would God command something which was objectively detrimental for his people?

It's one thing to try and argue the pros and cons on human terms, on what we think is important, but if God thought that a particular practice or ritual or tradition was good or bad, shouldn't that be important, especially to those who claim to worship and serve God? This isn't a rhetorical either, because even if we don't know all of the why, we still have God's nature and attributes to help us understand.

And they built the high places of Baal which are in the Valley of the Son of Hinnom, to cause their sons and their daughters to pass through the fire to Molech, which I did not command them, nor did it come into My mind that they should do this abomination, to cause Judah to sin.’ - Jeremiah 32:35 (NKJV)

God had strong opinions on what Israel was to, or not to, do. Modern Christians are not under the same law as the Hebrew people, but like Israel, we are the chosen of God and should consider his commands, even those historical, given for another people, to help show the pattern of God's behaviors. I can appreciate that some smart people throughout history have held a variety of different opinions on things like interest, or usury, if they separated the two in their discussions.

Look at what God recommended for Israel, for his people, and then try to figure out why God did that. Debate with folks on human terms is not a waste of time, but nobody is smarter than God, wiser than God, or has an understanding of how things all work out like God, so why not try to learn from what God has already done?