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 through believing in Jesus, you 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.
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 through salvation through Jesus, which brought it into being can never be earned by personal devotion or works of the Law. People get close to Jesus because His Spirit lives inside of the believer and so is always with them.
This personal relationship fulfills 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.
Catholic AnswerThe key principles of Christianity begin with Our Blessed Lord's first statement in the Bible:" Repent and believe in the Gospel." Mark 1:15. Without this initial repentance, none of the rest of it can follow. Our Blessed Lord then proceeds to make a mission statement about how the life of a Christian should look, this is covered in the Sermon on the Mount, chapters 5-7 of St. Matthew's Gospel, and finally, He gives us a charter on how we are going to be judged, as Mother Teresa's message to people was to love God. To love God, we must see Him in "His most distressing disguise" (her words): the poor, the homeless, the dying. And we must treat everyone of those people as if they were Our Blessed Lord, Himself. Because this is precisely how He told us we were going to be judged. Our salvation is dependent upon our faith and our works, see St. James 2:14-26: 14What good is it, my brothers, if someone says he has faith but does not have works? Can that faith save him? 15If a brother or sister has nothing to wear and has no food for the day, 16and one of you says to them, "Go in peace, keep warm, and eat well," but you do not give them the necessities of the body, what good is it? 17So also faith of itself, if it does not have works, is dead.
18Indeed someone might say, "You have faith and I have works." Demonstrate your faith to me without works, and I will demonstrate my faith to you from my works. 19You believe that God is one. You do well. Even the demons believe that and tremble. 20Do you want proof, you ignoramus, that faith without works is useless? 21Was not Abraham our father justified by works when he offered his son Isaac upon the altar? 22You see that faith was active along with his works, and faith was completed by the works. 23Thus the scripture was fulfilled that says, "Abraham believed God, and it was credited to him as righteousness," and he was called "the friend of God." 24See how a person is justified by works and not by faith alone. 25And in the same way, was not Rahab the harlot also justified by works when she welcomed the messengers and sent them out by a different route? 26For just as a body without a spirit is dead, so also faith without works is dead. and He has spelled out what those works are to be (St. Matthew's Gospel 25:31-46) 31 "When the Son of man comes in his glory, and all the angels with him, then he will sit on his glorious throne. 32 Before him will be gathered all the nations, and he will separate them one from another as a shepherd separates the sheep from the goats, 33 and he will place the sheep at his right hand, but the goats at the left. 34 Then the King will say to those at his right hand, 'Come, O blessed of my Father, inherit the kingdom prepared for you from the foundation of the world; 35 for I was hungry and you gave me food, I was thirsty and you gave me drink, I was a stranger and you welcomed me, 36 I was naked and you clothed me, I was sick and you visited me, I was in prison and you came to me.' 37 Then the righteous will answer him, 'Lord, when did we see thee hungry and feed thee, or thirsty and give thee drink? 38 And when did we see thee a stranger and welcome thee, or naked and clothe thee? 39 And when did we see thee sick or in prison and visit thee?' 40 And the King will answer them, 'Truly, I say to you, as you did it to one of the least of these my brethren, you did it to me.' 41 Then he will say to those at his left hand, 'Depart from me, you cursed, into the eternal fire prepared for the devil and his angels; 42 for I was hungry and you gave me no food, I was thirsty and you gave me no drink, 43 I was a stranger and you did not welcome me, naked and you did not clothe me, sick and in prison and you did not visit me.' 44 Then they also will answer, 'Lord, when did we see thee hungry or thirsty or a stranger or naked or sick or in prison, and did not minister to thee?' 45 Then he will answer them, 'Truly, I say to you, as you did it not to one of the least of these, you did it not to me.' 46 And they will go away into eternal punishment, but the righteous into eternal life."