"It has given me great joy to find some of your children walking in the truth, just as the Father commanded us." NIV translation
We see, here, that some of this woman's children were holding firm to Jesus Christ and some were not. John said it in a loving way but still points out that some of her children were not walking with the Lord. Love means that you tell someone the truth even when it is difficult for them to accept and it must have been difficult for this woman. This applies to the local bodies of believers (churches) of today as well in that many who enter the doors every week and profess to love God do not walk in the Truth which is Jesus Christ.
"And now, dear lady, I am not writing you a new command but one we have had from the beginning. I ask that we love one another. And this is love: that we walk in obedience to his commands. As you have heard from the beginning, his command is that you walk in love." NIV translation
In his first letter, we learned that we, as Christians, will be known by our love for each other.
True love is only possible for a believer in Christ because God is love and Jesus is the Way to God. We demonstrate our love for God by walking with Him (following His lead). If we are following Him, then we will also demonstrate His love to those around us.
"Many deceivers, who do not acknowledge Jesus Christ as coming in the flesh, have gone out into the world. Any such person is the deceiver and the antichrist." NIV translation
Now, John gets down to the tough truth which is the fact that people who deny that Jesus Christ is the Son of God are of the antichrist. This is the same thing that he told the church as a whole in his first letter.
"Watch out that you do not lose what you have worked for, but that you may be rewarded fully." NIV translation
John warns the believers of the danger in these people. He is not warning the believers that they could lose their salvation but is warning them that they could lose out on rewards as he describes that judgment in Revelation 20.