Because Lent is the season for introspection, self examination, and repentance, (of which I've been doing some of all three), I offer the following excerpt from Dietrich Bonhoeffer's "The Cost of Discipleship" for your consideration.
Much of what is ailing Christianity (and not just mainline protestantism; even if it is affecting/infecting us to a greater extent) can be explained by way of Bonhoeffers conception of "cheap grace". The fact that Bonhoeffer wrote this over 60 years ago is proof that this might be the single biggest challenge for Christians in general, but specifically mainline protestants in a modern/post-modern world:
"Cheap Grace is the deadly enemy of our Church. We are fighting today for costly grace. Cheap grace means grace sold on the market like cheapjacks' wares. The sacraments, the forgiveness of sin, and the consolations of religion are thrown away at cut prices. Grace is represented as the Church's inexhaustible treasury, from which she showers blessings with generous hands, without asking questions or fixing limits. Grace without price; grace without cost! The essence of grace, we suppose, is that the account has been paid in advance; and, because it has been paid, everything can be had for nothing. Since the cost was infinite, the possibilities of using and spending it are infinite. What would grace be if it were not cheap?
Cheap grace means grace as a doctrine, a principle, a system. It means forgiveness of sins proclaimed as a general truth, the love of God taught as the Christian "conception" of God. An intellectual assent to that idea is held to be of itself sufficient to secure remission of sins. The Church which holds the correct doctrine of grace has, it is supposed, ipso facto a part in that grace. In such a Church the world finds a cheap covering for its sins; no contrition is required, still less any real desire to be delivered from sin. Cheap grace therefore amounts to a denial of the living Word of God, in fact, a denial of the Incarnation of the Word of God.
Cheap grace means the justification of sin without the justification of the sinner. Grace alone does everything, they say, and so everything can remain as it was before. [......] Instead of following Christ, let the Christian enjoy the consolations of his grace! That is what we mean by cheap grace, the grace which amounts to the justification of sin without the justification of the repentant sinner who departs from sin and from whom sin departs. Cheap grace is not the kind of forgiveness of sin which frees us from the toils of sin. Cheap grace is the grace we bestow on ourselves.
Cheap grace is the preaching of forgiveness without requiring repentance, baptism without church discipline, Communion without confession, absolution without personal confession. Cheap grace is grace without discipleship, grace without the cross, grace without Jesus Christ, living and incarnate.
Costly grace is the treasure hidden in the field; for the sake of it a man will gladly go and sell all that he has. It is the pearl of great price to buy which the merchant will sell all his goods. It is the kingly rule of Christ, for whose sake a man will pluck out the eye which causes him to stumble; it is the call of Jesus Christ at which the disciple leaves his nets and follows him.
Costly grace is the gospel which must be sought again and again, the gift which must be asked for, the door at which a man must knock.
Such grace is costly because it calls us to follow, and it is grace because it calls us to follow Jesus Christ. It is costly because it cost a man his life, and it is grace because it gives a man the only true life. It is costly because it condemns sin, and grace because it justifies the sinner. Above all, it is costly because it cost God the life of his Son: "ye were bought at a price," and what has cost God much cannot be cheap for us. Above all, it is grace because God did not reckon his Son too dear a price to pay for our life, but delivered him up for us. Costly grace is the Incarnation of God.