How many books are there in the Bible? Most Protestants will tell you 66, but those who consider the Apocrypha (or Deuterocanonical books) part of the canon (along with the Old and New Testaments) will give you a number far higher. The Apocrypha contains books which some churches recognize as canon (divinely inspired by God), some churches recognize as peripheral, and others reject entirely. External arguments for why it should not be included, can be found here. External arguments for why it why it should can be found here. Interestingly, this topic was never mentioned in church when I was growing up (probably because neither Methodists nor Baptists acknowledge consider the Apocrypha to be divinely inspired). I list this difference first because before deciding what you believe, you should probably decide which books you believe in...

How is it that Christianity is considered a monotheistic religion? Most Christians will tell you there is God the Father (1 Corinthians 8:6), God the Son (John 10), and God the Holy Spirit (1 John 5:6-8). The Trinity is a doctrine that most denominations hold, which states that there are three divine beings (God the Father,  Jesus the Son, and the Holy Spirit) that co-exist in one Godhead.[22] However, not all denominations believe this.

There are actually a few beliefs among Non-Trinitarians. Non-Trinitarians generally believe that there are three separate beings, with some believing that all three beings are one in purpose, while others demote one or more from the status of God.  Because the Trinity doctrine is so widely accepted, this is a topic generally brushed over as a given, however, it is one of the major ideas that separates denominations and thus worthy of deep consideration before deciding which you believe. For more on about the differing beliefs about this doctrine, see this list of Non-Trinitarian views and this article on Trinitarian views.

A creed is a brief statement of belief.[15] The most popular creeds in Christianity are the Nicene Creed and the Apostles' Creed.

Some churches require belief in these creeds for membership to the church, while some merely recite them during service and many never recite them at all. The Methodist and African Methodist Episcopal (A.M.E) churches I attended growing up did recite creeds during church, though I don't think they are required for membership.[16] The Baptist church I attended never recited creeds.

John 3:16 says whoever believes in Jesus is rewarded by eternal life, but some denominations preach heavy emphases on good works are needed, furthermore declaring that salvation can be lost by evil deeds or lack of repentance. The Protestant movement is based on sola fide[19] which means “by faith alone.” Tangled up in the differing beliefs on salvation is the "once saved always saved" doctrine as well. Once saved always saved is most popular among Baptists. The road to salvation should definitely be at the top of the list for topics you should not just take for granted but actually study and affirm for yourself.

Speaking of salvation, the choice to believe in and accept Jesus belongs to us—or does it? Some churches believe that people are #TeamHeaven or #TeamHell (I couldn't resist doing that) from the start. The root of this belief, for many, is in Jeremiah 1:5. Some cite the 144,000 of Revelation as evidence of predestination, while other churches believe that you alone are responsible for your salvation. Whether you are a believer in predestination or not will probably affect the rest of your views as well.

Purgatory, according to Catholics, is a place where those who believed, but were impure go to be purified before ascending to Heaven. Most other denominations reject the idea of purgatory—it’s never mentioned in the Bible. Hell on the other hand is something almost all denominations agree on. However there are some (including myself) that reject the idea of hell being a place of eternal torment (see this wikipedia article on Annihilationism). In this view, the punishment of death by fire is final (eternal), but it doesn’t mean you burn for all eternity.

Another thing denominations can't agree on is what we should and should not consume or how we should or should not behave.

Some denominations preach complete abstinence from alcohol, while others allow real wine during communion. Denominations like the Latter Day Saints (Mormons) believe you should abstain from foods that contain caffeine as well. Some churches suggest vegetarianism. In determining what you believe in regards to diet (among most of the other topics mentioned), I would suggest looking at Jesus’ example—after all He is our perfect example of how to live. (Note: some people question whether the word wine in John 2 refers to the alcoholic beverage or grape juice, as Daniel 1:8 suggests consuming wine is to defile one's self, though it could be defiling because it was offered to a pagan god first).

In addition to diet, many denominations disagree on the attire/image of a believer. Some image related customs that are forbidden or frowned upon by certain denominations are as follows: wearing of jewelry, tattoos, women wearing pants, tight and/or revealing clothes, etc.