Behold, I will cast her into a bed - Not into a bed of ease, but a bed of pain. There is evidently a purpose to contrast this with her former condition. The harlot's bed and a sick-bed are thus brought together, as they are often, in fact, in the dispensations of Providence and the righteous judgments of God. One cannot be indulged without leading on, sooner or later, to the horrid sufferings of the other: and how soon no one knows.

And them that commit adultery with her - Those who are seduced by her doctrines into this sin; either they who commit it with her literally, or who are led into the same kind of life.

Into great tribulation - Great suffering; disease of body or tortures of the soul. How often - how almost uniformly is this the case with those who thus live! Sooner or later, sorrow always comes upon the licentious; and God has evinced by some of his severest judgments, in forms of frightful disease, his displeasure at the violation of the laws of purity. There is no sin that produces a mere withering and desolating effect upon the soul than what is here referred to; none which is more certain to be followed with sorrow.

Except they repent of their deeds - It is only by repentance that we can avoid the consequences of sin. The word "repent" here evidently includes both sorrow for the past, and abandonment of the evil course of life.

Revelation 2:22 Parallel Commentaries

That There is no Security against Temptation in this Life
"My Son, thou art never secure in this life, but thy spiritual armour will always be needful for thee as long as thou livest. Thou dwellest among foes, and art attacked on the right hand and on the left. If therefore thou use not on all sides the shield of patience, thou wilt not remain long unwounded. Above all, if thou keep not thy heart fixed upon Me with steadfast purpose to bear all things for My sake, thou shalt not be able to bear the fierceness of the attack, nor to attain to the victory …
Thomas A Kempis—Imitation of Christ

Job's Regret and Our Own
I. Let us begin by saying, that regrets such as those expressed in the text are and ought to be very BITTER. If it be the loss of spiritual things that we regret, then may we say from the bottom of our hearts, "Oh that I were as in months past." It is a great thing for a man to be near to God; it is a very choice privilege to be admitted into the inner circle of communion, and to become God's familiar friend. Great as the privilege is, so great is the loss of it. No darkness is so dark as that which …
Charles Haddon Spurgeon—Spurgeon's Sermons Volume 17: 1871

Of the Imitation of Christ, and of Contempt of the World and all Its Vanities
He that followeth me shall not walk in darkness,(1) saith the Lord. These are the words of Christ; and they teach us how far we must imitate His life and character, if we seek true illumination, and deliverance from all blindness of heart. Let it be our most earnest study, therefore, to dwell upon the life of Jesus Christ. 2. His teaching surpasseth all teaching of holy men, and such as have His Spirit find therein the hidden manna.(2) But there are many who, though they frequently hear the Gospel, …
Thomas A Kempis—Imitation of Christ