Normally, such a book written for believers would be absolutely unnecessary, for the Bible clearly says, "No one born of God commits sin; for God's nature abides in him, and he cannot sin because he is born of God" (1 John 3:9).
However, we know that a member of the church in Corinth did commit adultery and was disciplined. We also know that David fell into this same sin. The many warnings of the Apostle Paul to believers against adultery and fornication would leave the reader of the Epistles in no doubt that he wrote as one who knew that a converted person could fall into the sin of adultery and fornication.
It is common knowledge that in our generation, some of God's servants who, without doubt, have truly believed in the Lord Jesus, have fallen into adultery and fornication. So, a believer can be tempted, yield to temptation and commit adultery or fornication. If you want to be honest, you will acknowledge that you have come close to falling, or have actually fallen, in either thought, word or deed.
There are many who can say, "Thanks be to God, I have never committed adultery in act since I believed in the Lord Jesus." We thank the Lord for all such. That is only to be expected. However, the Lord Jesus is looking for those of whom He can say, "This one has never committed adultery in thought since he believed." I do hope He can say that of you.
Our prayer is that in the course of reading this book, you should know the deliverance of the Lord from the sin of adultery and fornication in thought, word and deed. The Lord Jesus, the Deliverer, is near to deliver you and to enable you to walk in that deliverance for the rest of your life until He comes.