All Christians believe the same key elements. The most basic and simple description of Christian beliefs is the Nicene Creed, written in 381 AD. This creed essentially says that Jesus, God, and the Holy Spirit all exist, are all one being, and have never created or non-existent; Jesus died on the cross for our sins so that we may have a relationship with God and enter into the eternal heaven upon death; and that everything in the Bible is 100% truth.
However, the minute and small details of the Christian faith tend to spark wars, divorces, church splits, immoral arguments, and prejudice amongst Christians. Some differences in personal beliefs include varying opinions on how to remember the Sabbath, Christian media vs secular media vs no media, baptism, songs sung in church, prayer, how church time and resources are spent, and methods of evangelism. While there have been no blood-thirsty wars started over the songs sung in church, many relationships and churches have been negatively impacted by fierce opinions over baptism, media, and other minor things. I cannot count the number of church splits I know of because of songs selected for worship. Believe it or not, I also know many arguments spun off of whether the preacher should use a pulpit, music stand, or table. Needless to say, there have been many petty and ridiculous arguments started by Christians.
One of my best friends and I also have our differences. To be specific, I believe that Christian rock music is awesome and capable of impacting many lives through the lyrics. My friend, however, believes that Christian rock music is bad because it was invented by non-Christians. My friend also believes that, for the most part, the only thing a Christian should do on Sunday (commonly believed to be the Sabbath, though it is actually Saturday) is go to church. I believe that Sunday is just another day of the week and shouldn't be respected any higher and I won't sit at home all day on Sunday just to be respectful. Unfortunately, it would be all too easy to get into a heated debate about our preferences, but that would obviously tear up our wonderful friendship. We both have to agree to disagree. Besides, we still serve the same God and love Him dearly. After all, there's nothing wrong with having different opinions and interpretations about God's commands. It's when I let those opinions come between me and a friend or me and my church that I start to sin.
My opinions, preferences, interpretations, and convictions all represent who I am. Other Christians will have different opinions, preferences, etc, because they are different than me. I cannot force my opinions on them or judge them for their opinions. EVEN IF my opinions are correct by the Bible and another Christian's opinions are not, I must not force him or her to believe as I do. He or she must grow to know truth their own way, certainly not by me shoving my opinions down their throat. That approach, more than any other, will turn people off from the truth, perhaps forever. In order to follow the commands of Jesus, we must love each other through our differences and mistakes. That is the most successful way to shed light into the darkness.
Romans 14:5-7 says, "One man considers one day more sacred than another; another man considers every day alike. Each one should be fully convinced in his own mind. He who regards one day as special, does so to the Lord. He who eats meat, eats to the Lord, for he gives thanks to God; and he who abstains, does so to the Lord and gives thanks to God. For none of us lives to himself alone and none of us dies to himself alone."