"Then the Interpreter took Christian by the hand and led him into a very large parlor that was full of dust because it was never swept. After He had … - John Bunyan

But now in this Valley of Humiliation poor Christian was hard put to it, for he had gone but a little way before he espied a foul Fiend coming over the field to meet him; his name is Apollyon. Then did Christian begin to be afraid, and to cast in his mind whether to go back, or to stand his ground. But he considered again, that he had no Armor for his back, and therefore thought that to turn the back to him might give him greater advantage with ease to pierce him with his Darts; therefore he resolved to venture, and stand his ground. For thought he, had I no more in mine eye than the saving of my life, 'twould be the best way to stand. So he went on, and Apollyon met him. Now the Monster was hideous to behold, he was cloathed with scales like a Fish (and they are his pride) he had Wings like a Dragon, feet like a Bear, and out of his belly came Fire and Smoke, and his mouth was as the mouth of a Lion. When he was come up to Christian, he beheld him with a disdainful countenance, and thus began to question with him.

Christian meets two more ill-fated pilgrims as he continues his expedition down the King's Highway. His ability to discern a false pilgrim has been advanced since his experience with Simple, Sloth, and Presumption.
Christian immediately focuses on three things that seem out of place. First, the tumbling over the wall called Salvation without coming through Christ, the narrow gate, or experiencing any illumination by the Holy Spirit immediately warns Christian that these are trespassers. Second, their testimony of having come from the land of Vain-Glory warns Christian that they are neither humble nor burdened by sin. Third, their wish to arrive at Mt. Zion to receive praise rather than to give praise to the only one worthy of praise alarms Christian. Christian quickly discerns that the motives of Formalist and Hypocrisy are unworthy and contemptuous.
Formalist

The law, instead of cleansing the heart from sin, doth revive it, put strength into, and increase it in the soul, even as it doth discover and forbid it, for it doth not give power to subdue.