If you are a parent, you don’t likely have much difficulty distinguishing your own kids from the neighbors’.
If the dad pulls into the driveway in the family minivan (or SUV, depending), and mom comes out and says “Dad, who are those children? They aren’t ours!” then you know dad has not discerned well (or he is some kind of criminal…).
The reason that parents can instantly recognize that foreign children are in the family SUV — except for those parents who tend to leave their children at various places, like they would their wallet or cell phone — is because they have spent so much time with them.
If they didn’t know their own children, Paul’s admonition in Galatians 1:6-9 might become “I’m astonished that you so easily forget which children you birthed, and fed, and clothed, to turn to others who don’t belong to you!”
But like parents who can discern their own children, and husbands who can discern their own wives, and art fans who can discern their favorite painter from others, disciples of Christ should be able to easily discern false gospels because they spend so much time with the true.
Disciples should be able to discern false gods because they spend so much time with the One True God.
Evaluate the time you spend studying the Bible, understanding the gospel, and communing with God, through Jesus, in the Spirit.
Does your time with the true prepare you to detect and reject the false? Are you sure? Repent of your neglect of knowing God through the gospel, and ask him to help you use your time better.