Jesus said an extraordinary and wonderful thing "For God, all things are possible."
Nothing is impossible for God. Yet we like to think that some things are impossible for Him. And one of our favourites is that God can only work through people like us. He can only work through our denomination. Or he can only work through the evangelical movement, or the protestant movement, or the catholic church. Or he can only work through people who are committed to the same doctrines that we are.
For God all things are possible. So it is not impossible that God would be working in other denominations. In fact we would expect God to be working in other denominations. If we read the parable of the lost sheep, or the prodigal son, we learn that God longs for reconciliation with his lost children and actively works to bring that reconciliation about. The shepherd goes out looking for the lost sheep. The father runs to meet his repentant son and hugs him. So does God actively work in all denominations? Of course he does.
The belief that God only works through people like us is a delusion - and a dangerous delusion. It is a mental illness. The body of Christ has a mental illness. In an individual it would be diagnosed as a narcissistic personality disorder - a disorder in which the victim thinks that they are right in all things and that anyone who disagrees with them must be wrong.
Mental illness stops us from being effective. It makes others wary of us. It makes others want to look the other way and pretend that they haven't seen us. It isolates us. This is the very opposite of what Jesus wants. Jesus tells us that it is by our love for each other that the world will know that we are his disciples. Love for each other cannot mean love that is restricted to people like us. It must mean love for all members of God's family.
As with any mental illness, we have to recognise that we are sick before we can be healed. We will not look for healing if we do not recognise and accept that we are ill. In the church, recognition and acceptance will lead to confession and repentance – and this will be the essential starting point for healing. Then we will know that members of other denominations are truly our brothers and sisters in Christ and we will love them. And then the world will know that we are Jesus' disciples.
"I give you a new commandment, that you love one another. Just as I have loved you, you also should love one another. By this everyone will know that you are my disciples, if you have love for one another." (John 13:34-35)