“But there were also false prophets among the people, even as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Lord who bought them, and bring on themselves swift destruction.”
Look at the word secretly. In this case, the word has the idea of laying something alongside something else. Laying something good alongside something that is bad. Very few false teachers and false prophets will directly deny the whole scope of Christianity. In fact, some of them will affirm what you and I believe. They might say, “Yes, we believe Jesus Christ is the Son of God.” Or they may say, “Yes, we believe that the Bible is the Word of God.” Or they might say “Yes, we ought to live good, godly, Christ-filled lives and so forth.”
But then they will secretly lay their false doctrine alongside that. The devil is very clever. The Bible says he’s “more cunning than any beast of the field” (Genesis 3:1). We are called to be on guard so that we won’t be deceived by partial truths and half gospels.
Spend some time in prayer, asking God to give you discernment and wisdom to recognize false teachings, even when they are brought in next to the truth.