All we need to look at, among the things we disagree with them about, is their salvation doctrine. An overwhelming high percentage say, "Just believe and you will be saved." You have to deny many scripture references on baptism to deny the necessity of it. But they do just that. The Galatians let a little Interdenominationalism in and look what the apostle Paul says about that.

Gal 1:1-10 "Paul, an apostle (not from men nor through man, but through Jesus Christ and God the Father who raised Him from the dead), and all the brethren who are with me, To the churches of Galatia: Grace to you and peace from God the Father and our Lord Jesus Christ, who gave Himself for our sins, that He might deliver us from this present evil age, according to the will of our God and Father, to whom be glory forever and ever. Amen. I marvel that you are turning away so soon from Him who called you in the grace of Christ, to a different gospel, which is not another; but there are some who trouble you and want to pervert the gospel of Christ. But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be accursed. As we have said before, so now I say again, if anyone preaches any other gospel to you than what you have received, let him be accursed. For do I now persuade men, or God? Or do I seek to please men? For if I still pleased men, I would not be a bondservant of Christ."
    Paul was an apostle of Jesus Christ with the authority from Christ to take the truth of the gospel to all. He rebuked the Galatians for turning from that Gospel. Interdenominationalism, at that time, was mixing a little Judaism and paganism in with the truth. Those who preached that false doctrine were condemned. Look what he writes to the Corinthians about that outside influence.
II Corinthians 11:2-4 "For I am jealous for you with a godly jealousy. For I have betrothed you to one husband, that I may present you as a chaste virgin to Christ. But I fear, lest somehow, as the serpent deceived Eve by his craftiness, so your minds may be corrupted from the simplicity that is in Christ."

    The gospel is so easy to understand but so many trip over it on their way to Interdenominationalism. Interdenominationalism today is much the same as it was then but the terms are changed slightly. It still hides the same old paganism and Judaism stirred into the mix. Sometimes the changes are so subtle that they are hardly noticeable. Paul writes to his beloved son in the faith about those who preach this false doctrine spread by Interdenominationalism. We see an insight into one of the reasons false doctrine prevails. 
I Timothy 6:3-5   "If anyone teaches otherwise and does not consent to wholesome words, even the words of our Lord Jesus Christ, and to the doctrine which accords with godliness, he is proud, knowing nothing, but is obsessed with disputes and arguments over words, from which come envy, strife, reviling, evil suspicions, useless wranglings of men of corrupt minds and destitute of the truth, who suppose that godliness is a means of gain.useless wranglings of men of corrupt minds and destitute of the truth, who suppose that godliness is a means of gain. From such withdraw yourself."
This false doctrine doesn't build up Christ and His church no matter how religious it sounds. This is one of the reasons that people are so negative about sounding religious. But let me ad that we are to be religious about our religion. Some called it at one time being a "fan". Just like when you go to the ballpark and root for the home teem. If we go root for the home teem religiously let's be just as religious about our religion. James makes mention of religion,         
(James 1:27) "Pure and undefiled religion before  God and the Father is this; to visit orphans and widows in their trouble, and to keep oneself unspotted from the world."    continued on next page