Christians should not invite any person who does not teach the truth about Christ into their homes. In the early church, Christians met for worship in homes (see Acts 2:46; 5:42; 8:3; 12:5, 12; Colossians 4:15; Philemon 1:2). Because of this, John may be referring to inviting false teachers into a meeting of the church.
This instruction may also refer to giving any type of hospitality to false teachers. Showing hospitality would support their work and help their message spread. John teaches that the only way to respond to them is to refuse to welcome them into the fellowship.