What are the key principles of Christianity?

Answer

The key principles of Christianity (i.e. the Kingdom of Heaven where Christ is King according to Scripture) are based on:
1. Humility or faith and trust in God
2. Communication with God through prayer and self-denial
3. Observance of The Law which is written in Scripture and in the hearts of those who love the truth
4. The offering of sacrifice to God and partaking of the sacrificial offering (various animal and bird offerings in the Old Testament, the Lamb of God in the New Testament, the sacrifice of the Mass in the Catholic Church).  

Answer

Belief in Jesus Christ and following Him in a personal relationship is the central principle under which all others come. This personal relationship, as well as the personal dealing with God in salvation which brought it into being can never be earned by personal devotion or works of the Law. The Law was done away with for Christians in terms of its saving power, or in order to get close to God. People get close to Jesus because His Spirit lives inside of the believer and so is always with them.

This personal relationship does away with a whole raft of ordinances which ultimately cannot please God or earn His favor. However, Christians are certainly called to obedience and faith and centrally to love as a first principle. This 'law of love' covers everything, especially the many situations in our modern age, never discussed in the New Testament.
Christianity is faith in Jesus Christ who died on the cross as a pentalty for your sins, so you could be forgiven. Once you are truly saved for eternity, you will want to obey God's word, which is the Bible. But the Bible isn't just a book of rules and laws, it has true accounts of real people's lives and how God made and impact on them.