I have put your two posts together, so that I could better answer them both.
1) The Bible clearly states that we cannot claim to be children of God and hate other Christians.
2) But it also says "love your neighbor as yourself," which is the second greatest command in the Bible.
3) I am just wondering why one of the apostles had to express the importance of loving other Christians as though it is different then just loving your neighbor as yourself?
Let me see if I can connect the dots for you.
Reading the verses you are referencing in the context of what was being said by John at the time.
John says.
1 Jn. 3:11
For this is the message that ye heard from the beginning, that we should love one another.
Where is that message?
John 13:34,35
34)
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.
1 Jn. 2:4,5
4) He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.
5) But whoso keepeth his word, in him verily is the love of God perfected: hereby know we that we are in him.
Example:
1Jn 3:12,13
12)
Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous.
13) Marvel not, my brethren, if the world hate you.
1 Jn. 3:15
Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him.
To say it simply, we (Christians) are a Minority in the world, we are hated by the world because we are not a part of the world.
John 15:18
If the world hate you, ye know that it hated me before it hated you.
Jn. 3:10
In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.
Therefore WE should look out for, and love the Brethren because, no one else will.
If we love our brothers and sisters who are believers, it proves that we have passed from death to life. But a person who has no love is still dead. Anyone who hates another brother or sister is really a murderer at heart. And you know that murderers don’t have eternal life within them.
1 John 3:14-15
This is the New Living Translation.
I think I answered it above.
Let me just say, John is known as the Love Apostle, that is why he and not the others put an emphasis on "The Love of the Brethren"
Paul puts it this way.
1Cor. 13:13
And now abideth faith, hope, charity, these three; but the greatest of these is charity....Charity is Love.
Hope this answers you question.
Be Blessed