2 John 1:4-6
"I rejoiced greatly to find some of your children walking in the truth, just as we were commanded by the Father. And now I ask you, lady, not as though writing a new commandment to you, but the one we have had from the beginning: that we love one another."
The old/new commandment: love one another. Not new in content — the same from the beginning — but always new in its application. Love and truth are inseparable in John: love without truth is sentimentalism; truth without love is legalism.
2 John 1:9-11
"Everyone who goes on ahead and does not abide in the teaching of Christ, does not have God. Whoever abides in the teaching has both the Father and the Son. If anyone comes to you and does not bring this teaching, do not receive him into your house or give him any greeting."
The doctrine of Christ as the dividing line: whoever remains in it has the Father and the Son; whoever deviates does not have God. The instruction not to receive false teachers into the home concerns hospitality as endorsement — not personal hostility.