The first point that must be addressed is the nature of Paul’s writing. Unlike the gospels, which do not have as clear an audience, the Epistles have a named audience (in this case, the Church in Rome). So, we must read his writings with this in mind. So, what issues might the church in Rome be facing.
Based on the full chapter, the people are arguing amongst themselves over the practices of their faith- honoring one day as greater than another (Sabbath) or dietary restrictions (eating meat, etc). It sounds like there is a conflict between Jews and Gentiles, and Paul is trying to bridge the gap that is forming. Thus, he is saying that as long as the practice is not against the will of God, or one that does not originate in faith, then there is no evil in it, and should not be condemned.
Now, how does this apply today. As Catholics, we are not expected to hold to the traditional Jewish (Kosher) diet. What Paul is saying today, for example, is that eating Kosher is OK if you are offering it as a sacrifice of love and faith, but not if it comes from a sense of superiority or pride. Eucharistic Adoration, the Tridentine Mass, and the Liturgy of Hours for the laity would be contemporary examples of this. People who practice these devotions should be allowed to do so, but not to the exclusion or defamation of others. This does not mean that such things as murder, abortion, premarital sex, or the like are “ok” if they are kept personal. Verse 23 confirms this: But he who has doubts is condemned, if he eats, because he does not act from faith; for whatever does not proceed from faith is sin.
As a general apologetic rule, always understand the context of a verse. Your brother uses a verse that sounds like one thing, but when read in context, sounds very different. I hope this helps.
