Some people see being a Christian as a private and individual thing.
Their idea is that you can be a Christian in your own way and by yourself. For them, Christianity is essentially about feeling good about themselves, as a form of self-actualisation and self-fulfilment. They are seriously mistaken. This kind of individualistic, modern understanding of what it means to be a Christian is not Biblical.
The fact that we are in relationship with God always means that we are in relationship with others. We cannot separate these two dimensions of our lives. This is well-illustrated in the metaphor Paul has used earlier in Romans 12:3–8: because we are the body of Christ, we are in intimate relationship with every other part of the body.
Every Aspect of our Lives Touches Others
Every aspect of our lives touches the lives of others. Paul has just been writing of the days on which you choose to worship and the food you choose to eat. If we claim freedom in Christ for ourselves, we cannot deny others their freedom. The pivot point here is our own relationship with Jesus. If we are mature in our relationship with him, then we will not impose our views on these kinds of matters on our brothers and sisters who are weaker in faith.
Take a sheet of paper and write the names of those people closest to you in the middle. Draw a circle around them. Then write the names of three of your closest friends outside that first circle, and draw a circle around them, also encircling the first circle. Then think of three more people who are close acquaintances and draw another circle around them. Then do this again for three more people who you know, but not that well. You should have four concentric circles, like a target. Then think carefully through this list, starting from the middle. How has your life impacted each of these people?