The word "denomination" comes from the same root word as denominator in fractions, which means to divide. Division is sinful. 1 Corinthians 1:10, 13 says, "Now I plead with you…that there be no divisions among you…Is Christ divided? Was Paul crucified for you? Or were you baptized in the name of Paul?" The doctrines of men caused division in the first century and they continue to divide today. The gospel of Christ taught, believed, and obeyed brings unity and eliminates denominationalism.
There is no salvation in denominationalism. Salvation is only in the body of Christ (Ephesians 5:23) which is His one church (Ephesians 1:22-23). Galatians 1:7, 9 says, "There are some that trouble you and want to pervert the gospel of Christ…if anyone preaches any other gospel to you than what you have received, let him be accursed." To get into a denomination one has to leave the teaching of the New Testament and do something different. Many people are going to be lost because of denominationalism (Matthew 7:22-23).
Every denomination has prostituted God’s plan of salvation with such things as "faith only" and "once saved always saved". They have also perverted the worship with instrumental music, speaking in tongues, fake healing, etc. 2 Peter 2:2 says, "And many will follow the destructive ways, because of whom the way of truth will be blasphemed." But we are warned in Colossians 2:18, "Let no man cheat you of your reward."
Because people want to do things their way 2 Thessalonians 2:11-12 says, "And for this reason God will send them strong delusion, that they should believe a lie: that they may all be damned who believe not the truth." We must not fellowship religious denominations in their error. 2 John 10-11 says, "If anyone comes to you and does not bring this doctrine, receive him not into your house, neither bid him God’s speed: for he that bids him God’s speed is partaker of his evil deeds." We sin if we fellowship religious denominations by becoming a partaker of their evil deeds. And as Ephesians 5:11 also says, "And have no fellowship with the unfruitful works of darkness but rather reprove them."