To hold onto sin in the mind, but not act on it is just as sinful as if one were to act out what is in the thought life. This is a teaching that is shunned, for the most part, today. I recently posted an article from Nick Roen, from Desiring God. In this article, Nick states God did not remove his desire for the same sex from him. This is nothing more than an excuse to continue on in his sinful, lustful ways.
There are a couple of verses that come to mind concerning the thought life, starting with Psalm 66:18, 'If I regard iniquity in my heart, the Lord will not hear'. Charles Spurgeon rightly states this, "If, having seen it to be there, I continue to gaze upon it without aversion; if I cherish it, have a side glance of love towards it, excuse it, and palliate it; “The Lord will not hear me.” How can he? Can I desire him to connive at my sin, and accept me while I willfully, cling to any evil way? Nothing hinders prayer like iniquity harbored in the breast; as with Cain, so with us, sin lieth at the door, and blocks the passage. If thou listen to the devil, God will not listen to thee. If thou refusest to hear God's commands, he will surely refuse to hear thy prayers. An imperfect petition God will hear for Christ's sake, but not one which is willfully mis-written by a traitor's hand. For God to accept our devotions, while we are delighting in sin, would be to make himself the God of hypocrites, which is a fitter name for Satan than for the Holy One of Israel."
The Hebrew for 'heart' is lêb; Brown-Driver- Briggs Hebrew Definitions defines it as 'inner man, mind, will, heart, understanding'. To 'regard' is 'to consider, look on, enjoy'. This is a willful act of entertaining sin in the inner man; something like pornography would feed sin in the thought life. Having lustful, sinful thoughts and entertaining those thoughts is sin, it also keeps God from hearing the prayers of all who do this.
It's worth looking into commentary from Albert Barnes as well, "literally, “If I have seen iniquity in my heart.” That is, If I have indulged in a purpose of iniquity; if I have had a wicked end in view; if I have not been willing to forsake all sin; if I have cherished a purpose of pollution or wrong. The meaning is not literally, If I have “seen” any iniquity in my heart - for no one can look into his own heart, and not see that it is defiled by sin; but, If I have cherished it in my soul; if I have gloated over past sins; if I am purposing to commit sin again; if I am not willing to abandon all sin, and to be holy."
'The Lord will not hear me' - "That is, He will not regard and answer my prayer. The idea is, that in order that prayer may be heard, there must be a purpose to forsake all forms of sin. This is a great and most important principle in regard to prayer. If there is still the love of evil in his heart; if he has some cherished purpose of iniquity which he is not willing to abandon; if there is any one sin, however small or unimportant it may seem to be, which he is not willing to forsake, he cannot hope that God will hear his prayer; he may be assured that he will not. All prayer, to be acceptable to God, must be connected with a purpose to forsake all sin." Albert Barnes
To cherish sin in the thought life is to keep yourself from being heard in prayer. The Bible does not command sinners to make excuses for sin, as Isaiah 55:7 states, " let the wicked forsake his way, and the unrighteous man his thoughts." This is a call to repentance, as Matthew Henry points out...
"There are two things involved in repentance: - [1.] It is to turn from sin; it is to forsake it. It is to leave it, and to leave it with loathing and abhorrence, never to return to it again. The wicked must forsake his way, his evil way, as we would forsake a false way that will never bring us to the happiness we aim at, and a dangerous way, that leads to destruction. Let him not take one step more in that way. Nay, there must be not only a change of the way, but a change of the mind; the unrighteous must forsake his thoughts. Repentance, if it be true, strikes at the root, and washes the heart from wickedness. We must alter our judgments concerning persons and things, dislodge the corrupt imaginations and quit the vain pretences under which an unsanctified heart shelters itself. Note, It is not enough to break off from evil practices, but we must enter a caveat against evil thoughts. Yet this is not all: [2.] To repent is to return to the Lord; to return to him as our God, our sovereign Lord, against whom we have rebelled, and to whom we are concerned to reconcile ourselves; it is to return to the Lord as the fountain of life and living waters, which we had forsaken for broken cisterns."
The word of God teaches us that we cannot cherish sin in our hearts, it must be forsaken and NOT excused. How horrifying that some would blame God for their own lustful thoughts!!
The prophet Jeremiah had a warning for the nation Israel; notice the words of Jeremiah, "O Jerusalem, wash thine heart from wickedness, that thou mayest be saved. How long shall thine evil thoughts lodge within thee?" Jeremiah 4:14
Sin is birthed in the thought life, in the inner man. It should be struck down right where it is birthed from, where it's first formed. No one says it better than John Gill concerning this text in Jeremiah, " These are the words of the prophet, or of God by the prophet, showing the cause of all their ruin and destruction, the wickedness of their hearts; and they are expressed in such form and language, as to be accommodated to the case of any unregenerate sinner: every man's heart is wicked, desperately wicked, even wickedness itself; everything in it is wicked; the thoughts, and the imagination of the thoughts of the heart, the mind, the understanding, the will, the conscience, and the affections; and everything that is wicked is in that: it is the womb in which all sin is conceived; the shop and forge in which it is wrought; it is the habitation of every unclean lust; the seeds and principles of all sin are in it; it is the fountain spring and source of all evil; of all evil thoughts, words, and actions; all come out of it, and have their rise in it: and this wickedness is of a defiling nature, and has left a pollution on it; and what comes out of it defiles the man, that he stands in need of washing; which cannot be done to purpose by ceremonial ablutions and sacrifices, by moral acts of righteousness, by humiliation and tears, nor by submission to Gospel ordinances; nor indeed is this to be done by man at all, any other way than by faith dealing with the blood of Christ, by which only the heart is purified: for this is God's work, as appears from his promises to cleanse his people from all sins; from their prayers to him, to create in them clean hearts, to wash them thoroughly from their iniquity, and cleanse them from their sin; from the sanctifying grace of the Spirit, and the washing of regeneration ascribed to him; and from the end and efficacy of the bloodshed of Christ, to cleanse from sin, and purge the conscience from dead works; and the design of such exhortations as these is to convince men of the wickedness and pollution of their hearts, of the necessity of being washed from it, and of their own inability to do it of themselves; and to lead them to the fountain of Christ's blood, to wash in for sin and for uncleanness."
Don't think the New Testament doesn't deal with the thought life, for Christ Himself teaches on it in Matthew 5:28; 15:19 and Mark 7:21. There is absolutely NO excuse for harboring sin in the thought life; all sin must be confessed and forsaken. There is no scriptural support for excusing sin in the heart. May God's word be honored, exalted, and obeyed among His people.
"Afresh, precious, precious Jesus, I resign this body to You, for doing or suffering, for living or dying. Will You accept it? Will You use me for Your glory more than heretofore, that You may have some little return for all the benefits You have done to me? Oh, do grant this request; my heart longs for it, my spirit pleads for it; and "if You will, You can." You know the hot temptation of which I am the subject. Bring Your glory out of it, and keep me from the evil, and it shall be well." - Ruth Bryan