2. Who is a brother or relative of Christ?
Answer: He who performs the will of God.

Proof:

Whosoever shall do the will of my Father which is in heaven, the same is my brother, and sister, and mother. (Matt. 12:50).

Ye are my friends, if ye do whatsoever I command you. Henceforth I call you not servants; (John 15:14-15).

3. What is it the will of God that we should do?
Answer: That we believe in His Son Jesus Christ and keep His commandments.

Proof:

This is the work of God, that ye believe on him whom he hath sent. (John 6:29).

And this is his commandment, that we should believe on the name of his Son Jesus Christ, and love one another, as he gave us commandment. And he that keepeth his commandments dwelleth in him. (1 John 3:23-24).

4. Who is it that has given the name "Christ's Brethren" to those who do the will of God?
Answer: It is Christ himself who has done so.

Proof:

He is not ashamed to call them BRETHREN, saying, I will declare Thy name unto my brethren (Heb. 2:11-12).

Go tell my BRETHREN that they go into Galilee, and there shall they see me. (Matt. 28:10).

Behold my mother and my BRETHREN (Mark 3:34).

...his Son, that he might be the firstborn among many BRETHREN (Rom. 8:29).

5. Why are the brethren of Christ called Christadelphian in our day instead of Christian?
Answer: Because multitudes are called Christian who do not believe the truth concerning Christ or obey his commandments. It has become, in fact, the name for a system of fable, and error that has risen up in the world in the place of truth, in fulfillment of the predictions of the apostles. Therefore, to be known as a "Christian" is not to be known as a believer of the truth.

Proof:

And they shall turn away their ears from the truth, and shall be turned unto fables. (2 Tim. 4:4).

Also of your own selves shall men arise, speaking perverse things, to draw away disciples after them. (Acts 20:30).

And many shall follow their pernicious ways; by reason of whom the way of truth shall be evil spoken of. (2 Peter 2:2).

For, behold, the darkness shall cover the earth, and gross darkness the people: but the LORD shall arise upon thee, and his glory shall be seen upon thee. (Isaiah 60:2).

And he will destroy in this mountain the face of the covering cast over all people, and the vail that is spread over all nations. (Isaiah 25:7).

With whom the kings of the earth have committed fornication, and the inhabitants of the earth have been made drunk with the wine of her fornication. (Rev. 17:2).

And the light of a candle shall shine no more at all in thee; and the voice of the bridegroom and of the bride shall be heard no more at all in thee: for thy merchants were the great men of the earth; for by thy sorceries were all nations deceived. (Rev. 18:23).