It's a common to hear the phrase, "All sins are the same in God's eyes". I've said it many times myself in the past. In reality, people will use this phrase as a means to show humility with their own sin so as to say, "How can I judge this person when I was once a sinner too". This posture is, in a sense, commended by Scripture. Consider Paul's words in Titus 3:1-3, "Remind them... to be gentle, and to show perfect courtesy toward all people. For we ourselves were once foolish, disobedient". We should not forget that we ourselves were sinners and yet God's grace came to us, thus, the Christian has no grounds to consider themselves higher than another as their salvation is entirely of grace. Yet, does this mean that all sins are equal in and of themselves? The Westminster Larger Catechism Q150-152 speak to this exact question. From the catechism we can answer this question... yes and no. Let's start with the yes.
The yes element is found in Q152 which reads,
Q: What doth every sin deserve at the hands of God?
A: Every sin, even the least, being against the sovereignty, goodness, and holiness of God, and against his righteous law, deserveth his wrath and curse, both in this life, and that which is to come; and cannot be expiated but by the blood of Christ.
The catechism shows its cards of the previous questions (which discuss the "no" aspect of my aforementioned answer to the question) when it says "Every sin, even the least". This, of course, assumes there is some sort of distinction that could be made between sins. Nevertheless, any sin, due to the fundamental nature of sin itself, incurs debt on the sinner and makes them deserving of the wrath of God. Whether I steal a candy bar (which I did a few times when I was younger) or I shoot my neighbor (I have not done that) I am deserving of God's wrath and in need of grace. In that sense, all sins are equal.
However, backing up in the catechism to Q150-151, we do see some distinctions. These distinctions are on levels of heinousness in sin. These two questions in the WLC are worth the price of admission alone. As catechism lists a detailed explanation of the various ways in which Scripture understands different sins to be greater in heinousness. Here is the list from Q151:
A: Sins receive their aggravations,
1. From the persons offending; if they be of riper age, greater experience or grace, eminent for profession, gifts, place, office, guides to others, and whose example is likely to be followed by others.
2. From the parties offended: if immediately against God, his attributes, and worship; against Christ, and his grace; the Holy Spirit, his witness, and workings; against superiors, men of eminency, and such as we stand especially related and engaged unto; against any of the saints, particularly weak brethren, the souls of them, or any other, and the common good of all or many.
3. From the nature and quality of the offence: if it be against the express letter of the law, break many commandments, contain in it many sins: if not only conceived in the heart, but breaks forth in words and actions, scandalize others, and admit of no reparation: if against means, mercies, judgments, light of nature, conviction of conscience, public or private admonition, censures of the church, civil punishments; and our prayers, purposes, promises, vows, covenants, and engagements to God or men: if done deliberately, willfully, presumptuously, impudently, boastingly, maliciously, frequently, obstinately, with delight, continuance, or relapsing after repentance.
4. From circumstances of time, and place: if on the Lord’s day, or other times of divine worship; or immediately before or after these, or other helps to prevent or remedy such miscarriages: if in public, or in the presence of others, who are thereby likely to be provoked or defiled.
The real deal is in the footnotes where each comment is referenced with Scripture. These footnotes are insane (you can read them here). It is certainly evident in Scripture that not all sins can be said as being equally heinous. But here's the thing... we know that already.
God has written his law on the hearts of all people (Rom 2:15). We have a God-given sense of (generally) what is right and wrong though this knowledge is in part suppressed by the natural man (Rom 1:18). With this, we have a God-given sense that some sins are more offensive than others. I used the example earlier of stealing a candy bar versus shooting someone when discussing how all sins deserve wrath. I use this analogy because of how ridiculous it is if we were to try and say that these two sins were equally heinous. To say that would be an injustice! God's Word demonstrates that and so does our sense of right and wrong.
Now, that isn't to say this phrase "All sins are the same in God's eyes" has no merit. As we have been able to witness, all sins are equally deserving of wrath and so in that sense the statement is true. However, perhaps we add more confusion than clarity when we speak this way.