Cults will tell you they are the "one true church". I belonged to one such organization for a decade as a young Christian. Such organizations allow members to have very few opinions about things, and define doctrine absolutely. In the church I belonged to, the founder of the organization basically spoke for God under a doctrine called "government of God". Disputing any doctrine seriously meant disfellowshipment.
Regarding denominations, evangelical churches, which include a wide variety of churches, hold the same core doctrines and are united on those.
Here's a list of those: the deity of Christ, the Triune nature of God (Trinity), substitutionary atonement, authority and inerrancy/infallibility of Scripture, virgin birth, original sin, justification by faith alone, the bodily resurrection of Christ and believers, and the coming judgment of the righteous and unrighteous.
My local fellowship belongs to an association (EFCA) but not a denomination. I like the idea of an association more than a denomination. The association churches work together in terms of evangelism, missions, and educational facilities, but the association does not control the local church fellowship, unless heresy is being taught, then they would be expelled. In addition, the local churches work collaboratively with other churches who do not belong to the association, and do not judge other fellowships on non-essential issues.
The above issues, though, are things we would hold with a tight fist, unwilling to yield on them whatsoever. Other things, like speaking in tongues, etcetera, are not essentials and we don't judge other churches on those things.
Regarding why denominations are not mentioned in the Bible, until the Reformation, the Roman Catholic Church and Greek Orthodox Church dominated the scene. Most denominations are a result of the Reformation. The RC Church controlled Christian doctrine for many years.
See attached chart.