I have just found these verses that have helped form my perspective. I have been wrestling with this issue for a long time and I have been doing a ton of research. I thank you for the opportunity to discuss them.
Romans 16:17-18
17 I urge you, brothers and sisters, to watch out for those who cause divisions and put obstacles in your way
that are contrary to the teaching you have learned. Keep away from them. 18 For such people are not serving our Lord Christ, but their own appetites. By smooth talk and flattery they deceive the minds of naive people.
While I have long been concerned about Jesus' prayer that Believers be one as he and the Father are one. I must say that this single section of scripture explains why it is ultimately dangerous to stress unity in the church with those who preach and practice a different Gospel than you understand yourself to be under. If you find your church is reading the bible in an alien way to you, it would seem best to bring things up with a pastor for discussion. Then to leave if it turns out you do have huge differences concerning the Gospel that will not be bridged.
Albert Mohler puts it well when he says that past issues of a different Gospel, there are issues of ecclesiology that can still divide and force people into different churches. One such issue near and dear to us is baptism. I for one would be unlikely to ever join a church which urges parents to baptize their children.
The problem here is that differences of Gospel and ecclesiology abound in the Christian faith.
However, we should certainly pursue a unity that transcends things like paint color of the sanctuary, types of carpet pattern for the church, and the musical genre of worship music. These kinds of divisions do destroy churches as far as I have heard. Those that do so face rebuke and discipline from God. That said, the real problem for me to be in unity with most in the Christian faith, with the vast, vast majority of those that proclaim themselves Christian; are differences in the type of Gospel given. In addition to core ecclesiological differences that would make me feel uncomfortable to join such churches.
Here I speak of the RCC, Eastern Orthodox Church, Mainline denominations, Seventh Day Adventists, Christian Science, Mormons, and Charismatic denominations and churches. There is simply no chance I would unite with such movements as they seem counter to the bible's teachings. They all have a different Gospel to the one I understand or an ecclesiology that is counter to what I think the bible teaches.
But, given evangelical culture's inroads to unity with the "global church," I would say that in light of the times I am definitely against unity if it means we have to sacrifice on key aspects of the doctrine we believe the bible teaches. That said, I am for unity with all those that hold to biblical teachings, the bible leaves me no choice but to seek unity.
In relation to doctrinal apostasy, it seems to come down to differences over Gospel and ecclesiology (the bulk of the NT teachings). Also, back in the 1st century this would have been a division between those that hold to what the apostles teach compared to those who add to or subtract from it.
