1 John 2:17 "And the world passes away, and the lust of it; but he who does the will of God abides for ever."

In modern usage the word lust almost always refers to intense, disordered sexual desire, but St. John is right that we can and do lust for worldly things in general. Anything that we desire with that all-consuming focus can be said to be lusted for, and lust is never good.

St. John contrasts lust for the world and worldly things with abiding forever in the will of God. "Abiding" implies calm steadiness, while lust is excited and temperamental. In order to become holy, then, we should not simply reorient our lust towards God or His will, instead we must, with God's help, renounce lust and change our way of thinking and our temperament.

We can sometimes fall into the trap of lusting for the faith, especially in the early stages of conversion. We can be all-consumed by our desire for God and pursue it at the expense of all else. Even this is not good. Lust always dies down eventually, and we have to be careful about what we will be left with when it does. Lust for anything, even God, can lead us to very bad behavior. We may dismiss, demonize, or hurt others who do not share our single-mindedness; we may be feeding our own ego and telling ourselves we are really doing God's will, thus making an idol even out of our faith.

There is a difference between lust and zeal, we should be discerning that we do not give in to lust and mistake it for zeal. I think St. Paul is a good example in this area. He was certainly on fire for God, but he did not let it lead him into the mistakes of lust. He preached with all of his heart and soul, but moved on when he was not accepted, and prayed for the conversion of all (see When We're Rejected by Our Own There's Still Work to Do). St. Paul never pursued God lustily, he never left the peace of abiding in God's love while doing His will. 

God is eternal and does not change. Doing His will should bring us peace and calm, even in the face of the sufferings and persecutions that are sure to come. The fruits of our behavior can help us discern whether we are controlling our lust. In his letters, and even by his life, St. John shows us what abiding in God's will looks like. John was peaceful, content in God's will, and that gave him the inner strength to be at the foot of the Cross; to abide in God's will even in the most difficult of circumstances.