They will: Peter uses the future tense in 2:1–3, but this does not mean that false prophets had not yet appeared. Instead, he is recalling the prophecy Jesus made that false teachers would arise (Matthew 24:11, 24; Mark 13:22; compare Acts 20:29–31; 2 Timothy 3:1–6). The false teachers already present in the church were the fulfillment of that prophecy.
denying the Master who bought them: The false teachers might have openly rejected Christ. But it is more likely that their bad behavior was a denial of Christ (compare Titus 1:16).