Most people whether they be Christian or not have heard of the ten commandments and could even name a few. However, they may not know that Jesus gave us a new commandment that sums up all of the ten commandments. If we obey this one commandment Jesus gave, we will surely obey everything else.
John 13:34 & 35: A new commandment I give unto you, That ye love one another; as I have loved you, that ye also love one another. 35 By this shall all men know that ye are my disciples, if ye have love one to another.
John also later wrote about this commandment:
1 John 2:7-11: 7 Brethren, I write no new commandment unto you, but an old commandment which ye had from the beginning. The old commandment is the word which ye have heard from the beginning. 8 Again, a new commandment I write unto you, which thing is true in him and in you: because the darkness is past, and the true light now shineth. 9 He that saith he is in the light, and hateth his brother, is in darkness even until now. 10 He that loveth his brother abideth in the light, and there is none occasion of stumbling in him. 11 But he that hateth his brother is in darkness, and walketh in darkness, and knoweth not whither he goeth, because that darkness hath blinded his eyes.
Jesus told us that people will know that we believe Him and uphold His characteristics if we love our brother. The term "brother" in this verse basically means everyone. God's children should have love towards all people no matter what they have done. Jesus says to love as He has loved us. Jesus has loved us unconditionally and even laid down his life for us. That is the kind of love we are to have for our fellow people.
John goes on to say in 1 John that if you don't love, you are in darkness (or sin). He also says that the darkness can blind us and make us not know where we are going. If you don't love others and have that sin in your life we can be blinded to even more sins. For example, you have to examine why you may not have Christ's love for someone. Are you jealous of what they have? Are you conventing their possessions? Have they done something hurtful to you? Jesus tells us we have to forgive as He forgave us (Matt 6:12).
Sometimes it is very hard to love someone because they may have hurt us or hurt someone we love more than what would seem forgivable. When we feel like we can never forgive or never love someone we need to ask the Lord to help us love as He loves. Jesus can love through us when we find it difficult.
Remember, how do unbelievers know that we are Christians? It is because of our Christ-like love towards others.