Thus you see, my dearest friends, that we live in foreign lands, while even our life is not our own, and we ought not to live to ourselves, and it requires great violence to seek by toil and to maintain by enthusiasm what a corrupted nature has not kept.
- Columbanus, Sermon X (Irish, 7th century)
"From the days of John the Baptist until now the kingdom of heaven has suffered violence, and the violent take it by force."
- Matthew 11.12
An alternate, and I believe better reading (cf. Dan. 2.44, 45) of that verb, "has suffered violence," is "exerts violence" or, as the ESV marginal note has it, "has been coming violently." When the Kingdom of God comes into a person's life, it turns over all the tables, exposes self-serving motives, and whips up on every sinful practice or desire.
It's called "conviction," and the Holy Spirit is all about conviction, in spades (Jn. 16.8-11). Conviction is like someone stepping in front of you as you're walking along. He blocks your path, gets up in your face, and pushes his finger into your chest.
This is what the Spirit does to everyone who enters the Kingdom of God. It's like saying, "Hey, what you doin' here? This is no way to act in the realm of righteousness, peace, and joy in the Spirit! Shape up! Now!"
We need to be as tough on ourselves if we're going to overcome our natural propensities toward self-seeking and self-serving. We'll have to work hard and whip up enthusiasm for Kingdom priorities, even as we learn to scorn and shed our old selfish ways. That'll take some serious self-pummeling: "it requires great violence to seek by toil and to maintain by enthusiasm what a corrupted nature has not kept."
Christianity is not greased skids, friends. We're in a war, and the enemy knows all our old ways and as-yet-unconquered-desires. And he'll come right at them. He'll offer you the world if you'll just indulge your old corrupted nature and not be too serious about the violence required to make progress in the Kingdom of God.
Are you enthusiastic about the Kingdom of God? Really? Would the people who know you agree? Are they frequently heard saying to someone else, "Man, that dude, you know, he's knocked-out crazy about that Kingdom of God thing"?
Because we will only toil, and sacrifice, and suffer pain for what we're serious about. Jesus, friends, is serious about His Kingdom. Look what He went through to bring it near. Jesus understands the violence required to seek and gain the Kingdom. He knows that Kingdom living is painful and violent, and He advises us to seek more of it, not less.
If you're not in this thing looking for a fight, you need to make sure you're in the Kingdom of God at all.