I would urge you to study Ephesians 4 ... it plainly says that the basis of Christian unity is maturity resulting in correct doctrine. Correct doctrine has EVERYTHING to do with unity.
11 And He gave some as apostles, and some as prophets, and some as evangelists, and some as pastors and teachers,
12 for the equipping of the saints for the work of service, to the building up of the body of Christ;
13 until we all attain to the unity of the faith, and of the knowledge of the Son of God, to a mature man, to the measure of the stature which belongs to the fullness of Christ.
14 As a result, we are no longer to be children, tossed here and there by waves and carried about by every wind of doctrine ...
Maturity in Christ, and doctrine, go together ... that is why Catholics and Christians will never unite ... mature Christians recognize the doctrinal errors of Romanism and are not willing to abandon the truth of scripture for unity under any conditions.
Can we agree on abortion, homosexuality, a few other issues? Yeah ... but the gospel of Jesus Christ is taught in the pages of the New Testament alone ... not in the writings, precepts, or rituals of Rome.
Yup. Couldn't agree more.*
But look: if you insist that you are only going to unite against Satanic attack with people who share exactly the doctrine that your church believes in, you're going to be in a pretty small group. There are thousands of separate Protestant denominations, all teaching different doctrines. Their adherents all think their church has it right. Logically, then, anybody who disagrees with the teachings of their denomination has it wrong. But since the Protestant denominations do differ in beliefs and practices, they can't all be right.
So who gets to decide? Who is really the best interpreter of Scripture? Honestly, I don't think I am. I'm not smart enough to have it all figured out. It amazes me when I encounter Protestants, Catholics, Jehovah's Witnesses, Mormons, or whatever who say, "I know better than all the rest of you, I'm much better at interpreting the Word of the Lord than you are, and after my own study of the Word I decided on this church. The rest of y'all are going to hell. My church, which was started a hundred years ago" (or 20 years ago, in many cases) "has it right and you don't. So we want nothing to do with you and we're going to fight the atheist left on our own."
It is not for us to judge the relationship between another person and God. Leave that to Him, Who knows all. They're His Scriptures. Join with your brothers and sisters in Christ. People of other denominations, Protestant, Orthodox, and Catholic alike, all love and serve the Lord. Don't give Satan the satisfaction of separating us. Divided we fall.
*Psssst: the particular verse you quoted is one Catholics often use to maintain that it wasn't the best thing for Protestant churches to introduce new doctrines after 1500 years of Christian Catholic unity. Just FYI. :-)