Why do so many Christians fail the 'practice what you preach' standard?

Christians have often heard the accusation "practice what you preach."
There are several reasons why Christians do not practice what they
preach or at least appear to others to not be practicing what they
preach.

First, not all who profess to be Christians really belong to Christ.
Jesus warned in His parable of the wheat and the tares that there
would be false believers in the world, sown by the enemy Satan, whose
aim is to cripple the testimony of Christians so that our witness to
the world would be compromised. Satan tries to destroy Christ's work
by placing false believers and teachers in the world who lead many
astray. One has only to look at the latest televangelist scandal to
know the world is filled with professing "Christians" whose ungodly
actions bring reproach on the name of Christ. In Matthew 7:22, Jesus
warned that many would profess faith but do not know Him. He also told
us we would recognize false believers by their fruit. A bad tree does
not produce good fruit, nor does a good tree produce bad fruit (Luke
6:43). Part of bad fruit is to not practice what you preach.

Second, we as Christians are not perfect. Christians are expected to
"practice what you preach," but we preach a message that is,
ultimately, impossible to live up to. While sinlessness is to be our
goal, it is not achievable in this lifetime. However, the fact that we
fail to live up to God's perfect standard does not change the fact
that it is to be our goal and message. Also, many times true
Christians behave in an un-Christian manner out of immaturity in the
faith or a lack of understanding of exactly what the Christian life
entails. New believers are often excited about their newfound faith
and eager to share it with others. When, because of immaturity, their
lives do not measure up to their testimony, charges of not practicing
what you preach are leveled at them. As we grow and mature in the
faith, our lives should more and more reflect the truths that we
profess to believe.

Fortunately, people are not saved by whether or not others practice
what they preach. Salvation is the gift of God through faith, and none
of those whom God has given to Christ will be snatched out of His hand
(John 10:28-29). No amount of false professions by unbelievers or
disappointing actions by immature Christians can change that.