Thanks for your question here. I think you’re right that Christians have been far too willing to divide over non-essential disagreements in the past. The existence of denominations actually show that people are trying to find the truth on all issues, and are often being too insistent that everyone agree with them in every respect. In Philippians 3:15-16 the Apostle Paul says “Let all who are spiritually mature agree on these things. If you disagree on some point, I believe God will make it plain to you. But we must hold on to the progress we have already made.” Notice what Paul is saying here – if people don’t agree on minor issues related to the faith, that’s ok. As long as they don’t forget “the progress we have already made,” or in other words the established core beliefs of Christianity. Romans 14:5,13,19 goes on to say “In the same way, some think one day is more holy than another day, while others think every day is alike. You should each be fully convinced that whichever day you choose is acceptable…So let’s stop condemning each other. Decide instead to live in such a way that you will not cause another believer to stumble and fall…So then, let us aim for harmony in the church and try to build each other up.”
That being said, I don’t think it’s necessary all sinful for us to disagree on things. We are in an age where we don’t yet understand everything. Now we see in a glass dimly, but then face to face. (1 Cor 13) Denominations aren’t the same as divisions. As my dad has said, denominations share a common denominator. Today there exists a friendly camaraderie between most denominations who are trying to serve the Lord in their own way.
Matthew 7:13-14 would definitely not be talking about denominations, in my opinion, but the difference between those who are unsaved and those who are saved. We’re not saved by having 100% correct theology, or even 50.0001% correct theology. Like you say, we’re all fallen, so it’s probably impossible for us to be completely right about everything. Rather, we’re saved by putting our faith in Jesus and accepting his free gift of salvation.
In short, I think we should take the approach of the old Christian saying In essentials unity, in non-essentials liberty, in all things charity.