I was asked this question recently...
...on the face of it, it is a good question.
The person who asked it was genuinely concerned with being obedient to Christ, and knowing God's commands.
So what's the problem?
A little context will help:
We were discussing racial politics in America, and the wisdom that scripture might offer to the church in such a muddy cultural milieu. Specifically, I was arguing that Christians ought to engage in cross-cultural relationships as a rule. This person was arguing that it is good for Christians to engage in cross-cultural relationships, but only when they are specifically called to do so; that it is also fine for Christians to only engage in relationships within homogenous peer groups.
I was arguing that it is a discipleship issue; to be mature in Christ, one must engage in relationships with other Christians who do not share your culture. I was arguing that it is an ethical issue: to be godly and good is to love our neighbors even though they are not like us.
At this point, the person asked, "Okay, that all sounds fine, but how do you justify that with scripture?"
There are two responses that I want to give to that question:
1) Why should I have to justify my position scripturally, but you don't? Why is it important for me to understand the biblical commands before engaging in cross-cultural community, but it is perfectly fine for you to maintain the status quo (ignoring people who aren't like you) without any need to know what scripture says about it?
2) Let's look at scripture! From Genesis to Revelation a major theme of the Kingdom of God is the bringing together of the nations under the rule and reign of our Lord and Messiah. The major ethical concern of the New Testament is precisely the creation of a new ethnos on the earth through the proclamation of the Gospel of the Kingdom of Jesus. Paul publicly rebukes Peter on exactly this point, to fail to eat with other Christians of different cultures is to fail to understand the Gospel. (Gal 2) Racism is the charge that must be dealt with by the Jerusalem church in Acts 6, and working out the sticky details of multi-cultural community is the primary focus of the first ecumenical council in Acts 15.
To come back to the point, it is right and good for scripture to be the proving grounds of new ideas. But scripture must also be applied to old ideas and patterns of life!
It is not enough to simply accept things as they are and move on. Jesus, our King, is redeeming all things to the Father, bringing all aspects of life under His dominion.