“An oppressed class which does not strive to learn to use arms, to acquire arms, only deserves to be treated like slaves. We cannot, unless we have become bourgeois pacifists or opportunists, forget that we are living in a class society from which there is no way out, nor can there be, save through the class struggle. In every class society, whether based on slavery, serfdom, or, as at present, wage-labor, the oppressor class is always armed. Not only the modern standing army, but even the modern militia - and even in the most democratic bourgeois republics, Switzerland, for instance - represent the bourgeoisie armed against the proletariat. That is such an elementary truth that it is hardly necessary to dwell upon it. Suffice it to point to the use of troops against strikers in all capitalist countries.
A bourgeoisie armed against the proletariat is one of the biggest fundamental and cardinal facts of modern capitalist society. And in face of this fact, revolutionary Social-Democrats are urged to “demand” “disarmament”! That is tantamount of complete abandonment of the class-struggle point of view, to renunciation of all thought of revolution. Our slogan must be: arming of the proletariat to defeat, expropriate and disarm the bourgeoisie. These are the only tactics possible for a revolutionary class, tactics that follow logically from, and are dictated by, the whole objective development of capitalist militarism. Only after the proletariat has disarmed the bourgeoisie will it be able, without betraying its world-historic mission, to consign all armaments to the scrap-heap. And the proletariat will undoubtedly do this, but only when this condition has been fulfilled, certainly not before.”

Nya Samhället (3 November 1913)
Sourced quotes

The Communist Manifesto, footnote (1888 edition)

1.2, Essential Works of Lenin (1966)
The State and Revolution (1917)

No Compromise – No Political Trading (1899)
Context: This foundation of the class struggle, which Marx – and this is his immortal service – has given to the modern labor movement, is the main point of attack in the battle which the bourgeois political economy is waging with socialism. The political economists deny the class struggle and would make of the labor movement only a part of the bourgeois party movements, and the Social Democracy only a division of the bourgeois democracy. The bourgeois political economy and politics direct all their exertions against the class character of the modern labor movement. If it were possible to create a breach in this bulwark, in this citadel of the Social Democracy, then the Social Democracy is conquered, and the proletariat thrown back under the dominion of capitalistic society. However small such a breach may be in the beginning, the enemy has the power to widen it and the certainty of final victory. And the enemy is most dangerous when he comes as a friend to the fortress, when he slinks in under the cover of friendship, and is recognized as a friend and comrade. The enemy who comes to us with open visor we face with a smile; to set our foot upon his neck is mere play for us. The stupidly brutal acts of violence of police politicians, the outrages of anti-socialist laws, the anti-revolution laws, penitentiary bills – these only arouse feelings of pitying contempt; the enemy, however, that reaches out the hand to us for a political alliance; and intrudes himself upon us as a friend and brother, – him and him alone have we to fear. Our fortress can withstand every assault – it can not be stormed nor taken from us by siege – it can only fall when we ourselves open the doors to the enemy and take him into our ranks as a fellow comrade. Growing out of the class struggle, our party rests upon the class struggle as a condition of its existence. Through and with that struggle the party is unconquerable; without it the party is lost, for it will have lost the source of its strength. Whoever fails to understand this or thinks that the class struggle is a dead issue, or that class antagonisms are gradually being effaced, stands upon the basis of bourgeois philosophy.

Source: The Manifesto of the Communist Party (1848), Section 2, paragraph 72 (last paragraph).

As quoted in The Military Programme of the Proletarian Revolution, Collected Works, Vol. 23, pages 78-9.
Attributions

Source: Why We Fail as Christians (1919), p. 27

Source: Reform or Revolution (1899), Ch.8

Collected Works, Vol. 10, pp. 83–87.
Collected Works