Marvel not - Do not think it so unusual, or so little to be expected, as to excite astonishment.
If the world hate you - The emphasis here is to be placed on the word "you." The apostle had just adverted to the fact that Cain hated Abel, his brother, without cause, and he says that they were not to deem it strange if the world hated them in like manner. The Saviour John 15:17-18 introduced these subjects in the same connection. In enjoining the duty of brotherly love on his disciples, he adverts to the fact that they must expect to be hated by the world, and tells them to remember that the world hated him before it hated them. The object of all this was to show more clearly the necessity of strong and tender mutual affection among Christians, since they could hope for none from the world. See the notes at John 15:18-19.

Marvel not - if the world hate you - Expect no better treatment from unconverted Jews and Gentiles than Abel received from his wicked and cruel brother. This was a lesson to the Church, preparatory to martyrdom. Expect neither justice nor mercy from the men who are enemies of God. They are either full of malice and envy, hateful, hating one another, or they are specious, hollow, false, and deceitful.
"A foe to God was ne'er true friend to Man."

Marvel not, my brethren, if the world hate you. By "the world" is meant the inhabitants of the world, the wicked part of them; these hate the saints, though without a cause, any just cause, and for no other reason, but because they are chosen and called out of the world, and do not live the wicked life they do: and this hatred of theirs is not at all to be wondered at; so it was from the beginning, and has been in all ages since; immediately upon the fall there was enmity between the seed of the woman and the seed of the serpent, which showed itself in Cain, the instance just given, who hated and murdered his righteous brother; Ishmael, that was born after the flesh, persecuted Isaac, that was born after the Spirit; and as it was then, it is now, the Jews persecuted the prophets of old, and hated Christ and his apostles. This is the common lot of all the saints, of all that will live godly in Christ Jesus; and therefore it should not be reckoned a strange and unusual thing; it always was so, even from the beginning, as soon as ever there were two sorts of persons, good and bad, righteous and wicked. This is a corollary or conclusion drawn from the above instance of Cain.

Marvel not--The marvel would be if the world loved you.
the world--of whom Cain is the representative (1-John 3:12).
hate you--as Cain hated even his own brother, and that to the extent of murdering him. The world feels its bad works tacitly reproved by your good works.

We know that we have passed from death unto life, etc. If our hearts are filled with brotherly love this shows that we are God's children; and the opposite is also true.
Whosoever hateth . . . is a murderer. Has the germs from which murder springs lurking in his heart. Hatred led Cain to murder; so it does many others who sin likewise.
No murderer. It is impossible for a man to have the spirit of murder in him and yet have eternal life.
Hereby perceive we the love. The love of God in Christ is meant. His love is shown on the cross. Christ is love. If we love as he loved, we must be willing even to die for each other.
But whoso, etc. If this be true, what shall be said of one who refuses to succor his suffering brother with all his goods?
Let us love not in word. Our love must not show itself in empty professions.

Marvel not if the world hate you - For the same cause.