Definition:
Internal sin refers to sinful thoughts, desires, and intentions that reside within the heart and mind, even if they do not manifest in outward actions. These sins are considered equally grievous in the eyes of God as external sins, as they reveal the true condition of the human heart.

Biblical Basis:
The concept of internal sin is deeply rooted in Scripture, where the focus is not only on outward actions but also on the inner thoughts and intentions of individuals. Jesus Christ emphasized the importance of the heart's condition in His teachings.

Key Scriptures:

1. Matthew 5:21-22 :
"You have heard that it was said to the ancients, ‘Do not murder,’ and ‘Anyone who murders will be subject to judgment.’ But I tell you that anyone who is angry with his brother will be subject to judgment."

Here, Jesus equates harboring anger with the act of murder, highlighting that internal emotions can be as sinful as physical actions.

2. Matthew 5:27-28 :
"You have heard that it was said, ‘Do not commit adultery.’ But I tell you that anyone who looks at a woman to lust after her has already committed adultery with her in his heart."

This passage underscores the sinfulness of lustful thoughts, equating them with the act of adultery itself.

3. Mark 7:21-23 :
"For from within the hearts of men come evil thoughts, sexual immorality, theft, murder, adultery, greed, wickedness, deceit, debauchery, envy, slander, arrogance, and foolishness. All these evils come from within, and these are what defile a man."

Jesus lists various sins that originate from within, emphasizing that internal sin defiles a person.

4. James 1:14-15 :
"But each one is tempted when by his own evil desires he is lured away and enticed. Then after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death."

James explains the progression from internal desire to sin, illustrating how internal sin can lead to external actions and spiritual death.

Theological Implications:
Internal sin challenges believers to examine their hearts and minds, recognizing that sin is not limited to visible actions. The Bible teaches that God judges the heart (1 Samuel 16:7), and thus, internal sins are subject to His judgment. This understanding calls for a deeper level of repentance and sanctification, as believers are urged to align their thoughts and desires with God's will.

Practical Considerations:
Believers are encouraged to guard their hearts and minds against internal sin through prayer, meditation on Scripture, and the guidance of the Holy Spirit. Philippians 4:8 advises, "Finally, brothers, whatever is true, whatever is honorable, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think on these things." This verse provides a framework for cultivating a heart and mind that are pleasing to God.

Historical Context:
Throughout church history, theologians have addressed the issue of internal sin. Augustine of Hippo, for example, emphasized the importance of inner purity and the need for divine grace to overcome sinful desires. The Reformers, including Martin Luther and John Calvin, also highlighted the pervasive nature of sin, including its internal aspects, and the necessity of faith and repentance.

Conclusion:
Internal sin is a significant aspect of Christian theology, emphasizing the need for believers to maintain purity of heart and mind. By recognizing and addressing internal sin, Christians can grow in holiness and deepen their relationship with God.Matthew 5:1-48
And seeing the multitudes, he went up into a mountain: and when he was set, his disciples came to him:
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Exodus 20:17
You shall not covet your neighbor's house, you shall not covet your neighbor's wife, nor his manservant, nor his maidservant, nor his ox, nor his ass, nor any thing that is your neighbor's.
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Matthew 5:27
You have heard that it was said by them of old time, You shall not commit adultery:
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Romans 8:1-39
There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit.
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Romans 12:2
And be not conformed to this world: but be you transformed by the renewing of your mind, that you may prove what is that good, and acceptable, and perfect, will of God.
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Romans 8:4
That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.
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James 1:13-15
Let no man say when he is tempted, I am tempted of God: for God cannot be tempted with evil, neither tempts he any man:
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Ephesians 5:5
For this you know, that no fornicator, nor unclean person, nor covetous man, who is an idolater, has any inheritance in the kingdom of Christ and of God.
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3709. orge -- impulse, wrath
... 3709 ("settled anger") proceeds from an internal which opposes someone or ... to God's) ,
controlled, passionate feeling against sin . . a indignation ...
//strongsnumbers.com/greek2/3709.htm - 7k

Whether Sin Has an Internal Cause?
... OF THE CAUSES OF SIN, IN GENERAL (FOUR ARTICLES) Whether sin has an internal cause?
Objection 1: It would seem that sin has no internal cause. ...
/.../christianbookshelf.org/aquinas/summa theologica/whether sin has an internal.htm

Whether the Devil Can Induce Man to Sin, by Internal Instigations?
... OF THE CAUSE OF SIN, AS REGARDS THE DEVIL (FOUR ARTICLES) Whether the devil
can induce man to sin, by internal instigations? Objection ...
/.../aquinas/summa theologica/whether the devil can induce.htm

Whether Sin Has an External Cause?
... 2]), the internal cause of sin is both the will, as completing the sinful act, and
the reason, as lacking the due rule, and the appetite, as inclining to sin. ...
/.../christianbookshelf.org/aquinas/summa theologica/whether sin has an external.htm

Whether the Sin of Morose Delectation is in the Reason?
... Consequently when the reason fails in directing the internal passions, sin is said
to be in the reason, as also when it fails in directing external actions. ...
//christianbookshelf.org/aquinas/summa theologica/whether the sin of morose.htm

Internal Foes.
... THE OVERCOMING LIFE. PART II. INTERNAL FOES. ... Paul says in his epistle to the
Romans:""For we know that the law is spiritual: but I am carnal, sold under sin. ...
//christianbookshelf.org/moody/the overcoming life/part ii internal foes.htm

Evidences Internal and Experimental.
... it is desirable in the present closing chapter to offer some suggestions pertaining
to the internal character of ... Thus it lays the axe at the root of all sin. ...
/.../barrows/companion to the bible/chapter xii evidences internal and.htm

Of the Causes of Sin, in Particular (Four Articles)
... We must now consider the causes of sin, in particular, and (1) The internal causes
of sin; (2) its external causes; and (3) sins which are the causes of other ...
//christianbookshelf.org/aquinas/summa theologica/of the causes of sin 2.htm

Whether the Sin of Consent to the Act is in the Higher Reason?
... Therefore the sin of consent to the act is not always in the higher reason. ... actions
according to the eternal law, so can he regulate his internal pleasures or ...
/...//christianbookshelf.org/aquinas/summa theologica/whether the sin of consent.htm

Of Internal Acts
... I intend here to speak only of Internal acts, those energies of the soul, by which ...
your heart" (Isaiah 46:8); for we have strayed from our heart by sin, and it ...
/.../guyon/a short and easy method of prayer/chapter xxii of internal acts.htm

Whether Idolatry is the Gravest of Sins?
... But in a case of external idolatry without internal unbelief, there is an
additional sin of falsehood, as stated above [3105](A[2]). ...
/.../aquinas/summa theologica/whether idolatry is the gravest.htm

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