COntext:
7 For (Q) many deceivers have (R) gone out into the world, those who (S) do not acknowledge Jesus Christ as coming in the flesh. This is (T) the deceiver and the (U) antichrist. 8 (V) Watch yourselves, (W) that you do not lose what we have accomplished, but that you may receive a full reward. 9 Anyone who goes too far and (X) does not abide in the teaching of Christ, does not have God; the one who abides in the teaching, he has both the Father and the Son. 10 If anyone comes to you and does not bring this teaching, (Y) do not receive him into your house, and do not give him a greeting; 11 for the one who gives him a greeting (Z) participates in his evil deeds.
Who are we not to receive?
Those that do NOT acknowldeg that Jesus Christ AS coming IN the flesh ( the incarnation). (Verse 7)
Those that do not have the (union of ) Father AND the Son. (verse 10).
IN short, we are not to receive anyone claiming to be a CHristian but that does NOT preach the incarnation ( and possibly resurrection, John isn't clear in that regards BUT from His other writing we can assume he also means that) or the union of Father and Son.