"Whoever will come after me, let him deny himself, and take up his cross, and follow me." Here our Lord speaks of what it means to be a true Christian: it is not merely right belief, but practicing the Faith in concrete ways. Indeed, it is easy to believe in Christ as Lord and Savior while living what is essentially a self-centered and worldly life, behaving for all intents and purposes as though He does not exist.
Truly, as St. Paul says, we are saved by faith, but genuine faith requires sacrifice and the willingness and desire to suffer, if need be, all things whatsoever on His behalf. While it is true that Christ died for our sins, His death on the cross does not constitute an automatic guarantee of our salvation. There is no such thing as a free passport to heaven. It is a true saying, as Scripture says, that "the Kingdom of God suffers violence, and the violent take it by force." That is, we must do violence (not against our fellow man, God forbid!) but against our fallen human nature.
Christ's death does not somehow let us off the hook. Rather, He died that we ourselves might be crucified with Him and say with St. Paul, "It is no longer I who live, but Christ livest in me." So long as our ego, with its selfish needs and desires, is the motivating force our our lives, we have not yet even begun to live a Christian life.
Of course we can, if we so choose, opt to pursue what the world considers to be the "good life," casting aside the burden and inconvenience of the cross. Yet even if we somehow succeed in gaining the whole world, there is a cost to be paid: the loss our our immortal soul. Truly, what gain is there in this? Who but a fool would prefer transitory pleasures and earthly treasures to the promise of eternal life in God's heavenly Kingdom?
Truly our soul's salvation is the one thing needful, the pearl of great price. This salvation, however, is neither an entitlement nor a right: it is the fruit of sacrificial suffering and an ongoing struggle to subdue the passions and to cleanse our hearts of sinful thoughts and desires.
This is the spiritual warfare to which we have all been called by virtue of our Baptism. It is never easy to follow this straight and narrow path, but the only alternative is eternal separation from God in a hell of our own making.