For the gospel to advance in any generation, it must be defended from those who would corrupt it.  Some people purposely corrupt the gospel to fulfill their personal agendas, whatever they may be.  Jude spoke of some of these who were present among first-century Christians.

Jude 1:4 (ESV)
⁴  For certain people have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ.

Peter referred to these false teachers as well.  

2 Peter 2:1 (ESV)
¹  But false prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them, bringing upon themselves swift destruction.

Peter indicates that there will always be false teachers among the people of God.  History has proven this to be true.  Paul suggested that in his day there were already many who were corrupting the Word of God (2 Corinthians 2:17), and with many tears, he warned brethren about them (Acts 20:31).  It was for this reason that he was ready to defend the gospel against all attempts to defile it.

Philippians 1:15-16 (ESV)
¹⁵  Some indeed preach Christ from envy and rivalry, but others from good will. ¹⁶  The latter do it out of love, knowing that I am put here for the defense of the gospel.

We must not only preach the gospel, we must defend it as well.  Tolerating error, and those who propagate it, slows down the progress of the gospel, and in some cases brings it to a screeching halt.  This happens when it is allowed to penetrate the fortress of God’s house.  It is our responsibility as Christians, and as the church, to expose error and fight for the truth.  This is implied by the description Paul gave to the church.

1 Timothy 3:15 (NKJV)
¹⁵  but if I am delayed, I write so that you may know how you ought to conduct yourself in the house of God, which is the church of the living God, the pillar and ground of the truth.

Being the pillar and the ground of the truth, we must support the truth at all costs, something we cannot do if we are not set for the defense of the gospel.  Defending the gospel is usually unpopular.  There is an ecumenical climate in the denominational world that leads people to believe that doctrinal differences do not matter and that we should all agree to disagree on issues where we are divided, as long as we believe in Jesus.  Anyone who defends the faith against doctrinal innovations is deemed to be narrow-minded and unloving.  What should we do in such a climate?  Jude tells us.  We must earnestly contend for the faith (Jude 1:3).

If we want the gospel to advance, we must protect it against all perversions.  It will not always be popular to do so, but it will always be right.  

As you wind down for the night, think about these things.