There are a number of answers to this question, including praying for Christians around the world, and taking care of other Christians we know in times of need. But one of the ways we do this is by working together formally in denominations. Different Church denominations have different institutional structures, but the basic idea is for different churches to be able to support each other and hold each other accountable. The overall difference in structure is how the independence of the individual church is balanced with the accountability to the other churches in the denomination.
One example of how this works is that we only appoint pastors who are approved by the other WPC pastors of WA.
Of course one of the dangers of denominations is to think that the churches in our denomination are the only true churches. Both Three Crosses Church and Westminster Presbyterian Churches as a denomination are committed to partnering with churches of other denominations as much as we can.