Dear Marie:

The reason the Kingdom is under more severe attack is because after John the Baptist comes the New Covenant of Christ. Christ's death and resurrection is the redemption of all mankind. The devil, therefore, intensifies his efforts.

Here is the commentary from the Navarre Bible. This commentary may seem a little hard to understand. I will try to simplify it at the end:

Once John the Baptist announces that the Christ is already come, the powers of hell redouble their desperate assault, which continues right through the lifetime of the Church (cf. Eph 6:12). The situation described here seems to be this: the leaders of the Jewish people, and their blind followers, were waiting for the Kingdom of God the way people wait for a rightful legacy to come their way; but while they rest on the laurels of the rights and rewards they think their race entitles them to, others, men of violence (literally, attackers) are taking it, as it were, by force, by fighting the enemies of the soul -- the world, the flesh, and the devil.

"This violence is not directed against others. It is  a violence used to fight your own weakness and miseries, a fortitude, which prevents you from camouflaging your own infidelities, a boldness to own up to the faith even when the environment is hostile" (St. Escrivá, Christ is passing by, 82).

This is the attitude of those who fight their passions and do violence, thereby attaining the Kingdom of heaven and becoming one with Christ. As Clement of Alexandra puts it: "the Kingdom of heaven does not belong to those who sleep and who indulge their desires, but to those who fight against themselves" (Quis dives salvetur, 21).

What all that means is that the devil re-doubled his efforts once it was announced that Jesus Christ, the Messiah, had come. This was a great threat to the devil and he responded accordingly.

The violence referred to here is that which comes when men attack the passions within themselves, fight the enemies of our soul -- the world, the flesh, and the devil. By mortifying (putting to death) the disordered passions within us we are attaining the Kingdom of God and becoming one with Christ. In this we "take by force" the Kingdom away from the coach potato Jews who were passively awaiting the Coming of the Messiah. As St. Clement said: "the Kingdom of heaven does not belong to those who sleep and who indulge their desires, but to those who fight against themselves."

I hope this helps.

God Bless,
Bro. Ignatius Mary