a lot of beliefs central to Christianity aren't found in scripture).
If we're talking about traditional, modern-day "Christianity",
then I have a fairly large list ( that only begins to touch on Roman Catholic beliefs, for example ):
Dressing up on Sundays to assemble together with the local body of Christ,
"Clergy / Laity" distinction,
Tithing as opposed to giving ( Old Testament temple support versus true Christian giving ),
The assembly being a "spectator sport" where one man or group directs most of what happens during it, instead of everyone exercising their spiritual gifts together,
A "worship band",
Everyone going along with what the pastor says about a given subject, without checking it against what God's word says,
"Pews",
An "altar" at the front of the church ( part of Hebrew Old Testament temple sacrifice ),
A "baptistry" ( baptisms were usually performed in lakes and rivers, and still are in some parts of the world )
"Bible colleges" and "seminaries",
A "choir",
" Vestments",
Pastors and other members who make a paycheck for serving the local assembly with their spiritual gifts,
"Trustees",
A parsonage,
A church building or meeting house,
"Christmas" or any other holiday,
...many more.
In fact, much of this list has become central to most people's understanding of how Christian churches are supposed to be established and operated, when very nearly none of this was practiced or even thought of in the first century;
It just became traditional over the course of the past 2,000 years, some of it within the past 50 to 100 or so.
For example, "Christmas" wasn't celebrated for the first few hundred years after Christ ascended, and in America it wasn't celebrated until sometime in the early 1800's.
It was not even observed as a federal holiday until 1870.
"Clergy-Laity distinction" is probably the oldest one not found in the Scriptures,
unless one considers having a single pastor ( or team of them ) as being similar to the Old Testament model in which the pastor replaces the priest and / or prophet of God...
but since everyone in the body of Christ is a prophet, priest and a king, then there really is no reason to elevate anyone above another.
Respect for an elder?
That's biblical and that's different.
Just some thoughts off the top of my head, when I compare the Scriptures with what I see happening today.