Do Protestants believe that all sins are equal? I know that Protestants reject the Roman Catholic understanding of "mortal" and "venial" sins. I also know that there's biblical basis for seeing sins as both equal and different.  

But how do Protestants explain this?  According to them, are sins equal or different?

Related: How do Protestants understand the &quot;unforgivable&quot; sin?
The consensus among Protestants is that some sins are worse than others, but even the smallest sin deserves God's wrath.  Or, put another way, sins are equally damning, but not equally heinous.  The Westminster Shorter Catechism puts it succinctly:

Every sin deserves God's wrath and curse, both in this life, and that which is to come. (Answer 84)
Some sins [...] are more heinous in the sight of God than others. (Answer 83)

Reformed theologians like Louis Berkhof1 and John Frame2  follow this framework.  So do Baptists like A. H. Strong3 and Wayne Grudem.4  Grudem explains:

In terms of our legal standing before God, any one sin, even what may seem to be a very small one, makes us legally guilty before God and therefore worthy of eternal punishment.
On the other hand, some sins are worse than others in that they have more harmful consequences in our lives and in the lives of others, and, in terms of our personal relationship to God as Father, they arouse his displeasure more and bring more serious disruption to our fellowship with him.

Arminian theologian H. Orton Wiley5 similarly holds that the penalty of sin is &quot;death physical, temporal and eternal,&quot; but that the term &quot;penalty [...] must be made to include the consequences of all the various evils included in sin.&quot;  He thus describes &quot;degrees of both guilt and penalty as in the case of sins of ignorance or infirmity as over against sins of knowledge.&quot;
Anglican W. H. Griffith Thomas6 explains that &quot;[a]ll sin is deadly in that it tends toward death, but there are sins which because they are deliberately committed against light are obviously more injurious to the soul.&quot;
Lutheran theologian John Mueller, condensing Franz Pieper's work,7 likewise writes:

Every transgression of the divine Law is rebellion against God and therefore damnable, Gal. 3, 10.  From the viewpoint of damnability therefore we cannot speak of &quot;smaller&quot; and &quot;greater&quot; sins.  Still Scripture itself distinguishes degrees in sinning [....] [and] degrees also in the eternal punishment which the damned will suffer.


References

Berkhof, Systematic Theology, 2.2.4, p252
Frame, Systematic Theology, Chapter 36, p851.
Strong, Systematic Theology, Volume 2, Part V, Chapter VI, pages 656 and 648-650
Grudem, Systematic Theology, Chapter 24, D-4, p501.
Wiley, Christian Theology, Chapter 18
Thomas, Principles of Theology, p230-31
Mueller, Christian Dogmatics, p230