A Hymn to God the Father The Door to Sin

After the speaker sins, other people make his sin "their door" to more sinning. He's using a metaphor. By comparing sin to a doorway by which other sins can be entered, he's saying that he is guilty of holding that door open every time he sins. He thinks it is his fault that others saw him sinning and decided that they, too, should sin… kind of like how seeing a celebrity smoke might make smoking seem cool to their fans.