I am reading†the book of Jude and he said in verse 3, "Beloved, when I gave all diligence to write unto you of the common salvation, it was needful for me to write unto you, and exhort [you] that ye should earnestly contend for the faith which was once delivered unto the saints."
What exactly does it mean to contend for the faith?† Does it mean when we are talking to others who have said that they are Christians but believe in things that are not scriptural or does it mean just talking to those who are members of the church we attend that have decided to follow other teachings not in the Bible?
"Beloved, while I was very diligent to write to you concerning our common salvation, I found it necessary to write to you exhorting you to contend earnestly for the faith which was once for all delivered to the saints. For certain men have crept in unnoticed, who long ago were marked out for this condemnation, ungodly men, who turn the grace of our God into lewdness and deny the only Lord God and our Lord Jesus Christ" (Jude 3-4).
By adding in the context we see that Jude is urging Christians to do battle to keep the faith as it was originally delivered against the false teachers who had already begun to appear among the disciples and changing the truth for lies.
"I marvel that you are turning away so soon from Him who called you in the grace of Christ, to a different gospel, which is not another; but there are some who trouble you and want to pervert the gospel of Christ. But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be accursed. As we have said before, so now I say again, if anyone preaches any other gospel to you than what you have received, let him be accursed. For do I now persuade men, or God? Or do I seek to please men? For if I still pleased men, I would not be a bondservant of Christ" (Galatians 1:6-10).
Whether the influence for falsehood comes from within or without, the need to remain faithful to the truth remains the same.
"And consider that the longsuffering of our Lord is salvation -- as also our beloved brother Paul, according to the wisdom given to him, has written to you, as also in all his epistles, speaking in them of these things, in which are some things hard to understand, which untaught and unstable people twist to their own destruction, as they do also the rest of the Scriptures. You therefore, beloved, since you know this beforehand, beware lest you also fall from your own steadfastness, being led away with the error of the wicked" (II Peter 3:15-17).