Jesus prayed that future believers would be united to a humanly impossible degree (John 17:20 – 26). So, denominationalism is not acceptable – but it exists so we need to have a way to deal with it.
Ephesians 4:1-6 emphasizes the same point. However, this list should not be used as the “essential” doctrines because each group has a different way of looking at each item in the list. So, each group dismisses the others.
I cannot assume that my views are correct, and further assume that anyone who disagrees with me is wrong. Certainly, I will not keep a doctrine after I figure out my mistake, so any doctrine I hold at this moment I assume to be correct. But, I must also know that growth is essential (2 Peter 1:2 – 11). If I am growing, then what I learned today means that I had something wrong yesterday, and what I learn tomorrow will show that something I believe will be shown to be wrong.
So, I must approach others as equals. I must let them explain their position and agree where I can. The important attitude is to try to help the other person make his argument work. Any fool can support his own doctrine. Then, I must, with all kindness and patience, explain my position. I cannot demand instant obedience to my ideas. It took me years to get where I am, so I cannot expect others to get there instantly.
Focus on the good stuff (Philippians 4:8). Examine everything carefully; keep the good (1 Thessalonians 5:21). Focus on the unseen, not the seen (2 Corinthians 4:18, 5:16).
Concerning denominationalism, obviously, that is a big failure for us. However, we must remember that God is very intelligent. God set up the church and left it in the hands of people. God did not need to be a rocket scientist to know that we were going to make a mess of this. God can work with it.
Finally, God adopts individuals into His family, not groups. So, we cannot say that this group or that group is acceptable, but some other groups are not. What we can say is that, because of the weaknesses in this or that doctrine, individuals who have been poorly taught are more likely to fail in their faith (1 Corinthians 3:11 – 15). We can be certain that the difficulties of life will hit us right where our understanding is most poorly constructed, which may collapse the whole spiritual house which each of us is.