Paul's use of "therefore" indicates that this is a conclusion he has drawn from what he has taught. He spent the largest part of this letter (chapters 1-11) teaching about the gospel and God's plan of salvation. He then moved to application and showed how a transformed person will live differently (chapters 12-14).
But the most difficult place for transformation is in the church itself. There is a reason Paul wrote about transformation to the Romans, and why his gospel presentation in chapters 1-11 detailed the way that God has made salvation available for all. All have sinned, all are in need of God's grace, and God offers his grace freely through Christ, apart from the Law.
We have been welcomed by Christ. Therefore, we ought to put aside our differences with others, and the ways we exclude them from our fellowship, and welcome them. This brings glory to God.
A church is welcoming when members welcome others the way Christ welcomed them.
The good news is exclusive because it is available only through Christ, but it is inclusive because it is for all. When we situate church buildings away from the population we need to serve, we are not welcoming them. When we discourage people from being involved because they don't fit our mold, we are not welcoming them. When we frown on children being a little disruptive during worship, we are not welcoming.
One of the most unwelcoming things some churches do is to make families with young children feel uncomfortable and unwelcome. This occurs in a variety of ways: when parents are given looks during worship that are meant to indicate their child is disruptive; when comments are made to the effect that it is the parent's "turn" to teach their own children's Bible classes; and when announcements are made to the effect that children being children (i.e., running through the building) is both disruptive and a problem.
These things may be a legitimate problem in some places, but there are welcoming and unwelcoming ways of handling them. Jesus even had to rebuke his disciples who wanted to send children away from him. He welcomed the children, and said that the kingdom of God belonged to such as they (Matthew 19:13-15).
Our unwelcoming ways are subtle. But if we drill down deep enough, we can discern whether we are welcoming or unwelcoming by whether our decisions and choices would bring glory to God. If someone ends up being excluded from the inclusive gospel, there is no glory given to God. When we break out of our unwelcoming ways, offer hospitality to those in need, and welcome those who are different than us, we bring glory to God.