How can food be unclean to anyone who thinks it unclean?

Paul has to convince his readers of the liberty he advocates—are they not required to communicate to the weak person that all food is clean? Certainly, but this conviction may never become something that causes the weak believer to stumble: I know and am persuaded in the Lord Jesus that nothing is unclean in itself, but it is unclean for anyone who thinks it unclean (Romans 14:14). When it concerns convictions, Paul purposefully chooses the position of the omnivore. However, this personal conviction may not work for the weak in faith, because they still regard some foods as unclean. While on earth, Jesus already indicated that it is not the actual food that makes a person unclean, but rather everything that comes out of his mouth (and his heart). As such he declared all food to be clean (Mark 7:14–23). For a while the apostles still remained weak in this faith, which meant that Jesus had to later command his apostle Peter in a vision to eat that which he still regarded as unclean (Acts 10:1–48). Then Jesus reminded him that God had already declared all food to be clean: What God has made clean, do not call common (Acts 10:15). We can assume that the Christians in Rome to their fellow Christians in the same manner. However, the difference lies in the fact that while the Lord who prepared his servant Peter for a particular role as leader, Christians must, when interacting among one another, protect each other even in their weakness. This is also why the same Paul, who called Peter to order in Antioch when the latter reverted to avoidance of unclean food (Galatians 2:11–14), can provide support to the Jewish Christians in Jerusalem who still adhere to the dietary laws (Acts 21:15–26). Furthermore, he can also exhort the Gentile Christians in Rome to serve one another even while maintaining their good opinions as opposed to causing those who are afraid to each meat or drink wine to fall.¹