People want to belong to a group. We are programmed as humans to belong. People sort themselves into various groups: climbers, hikers, bikers, hunters, etc..
Even though we naturally sort ourselves into groups it doesn't mean that we feel like we belong. Mother Teresa said years ago in an interview that the world's greatest problem was loneliness. People are lonely, despite the fact that we have more ways to communicate than ever before.
As Christians we should first welcome people into our fellowship before we force them to believe or behave in ways that we think they need to. Jesus went to the people on the fringes, he met them where they were and after their encounter with him they began to change.
We need to trust that if Christ is in our midst that people will encounter Him and by encountering him begin to believe in Him and in his message.
The second "B" is believing. It is the context of belonging that people begin to ask questions, seek answers and begin to believe. This is why the Alpha course stresses hospitality first and foremost. As Christians we should model radical hospitality that recognizes the inherent value of each person based on their being created in the image of God.
The third "B" is behaving. After people have encountered Christ through radical hospitality and love, been able to work though their intellectual issues then they will begin to behave in ways that are consistent with gospel living.
So often we start with believing, then move to behaving and finally based on right belief and behavior allow people to belong. This is totally the wrong order. We need to recover a more Christlike way of welcoming people.