R.C. Sproul has written,

Protestantism affirms that all sin is serious.  Even the smallest sin is an act of rebellion against God.  Every sin is an act of cosmic treason, a futile attempt to dethrone God in His sovereign authority. (Essential Truths of the Christian Faith, 1st edition, page 144)

Or Herman Bavinck,

As extensive as original sin is in humanity as a whole, so it is also in the individual person.  It holds sway over the whole person, over mind and will, heart and conscience, soul and body, over all one’s capacities and powers. (Our Reasonable Faith, pg. 243)

Though always hugely unpopular, the personal reality of sin must be confronted.  We do not do this as “holier-than-thou’s” but as sinners ourselves, daily facing that awful fact.

Sin should be realized before the presence of God – it must bring us before Him, Who is of “purer eyes than to behold evil” (Habakkuk 1:13). This God is not merely Law-Giver and Judge, He is also Creator, Redeemer, Sanctifier and Lord!

We must also see our sin by the standard of God’s Law, remembering that it is an uncovering act of God’s grace.  1 John 1:8-10 is a clear assertion of the un-covering of ourselves confessing our sin so that God may cover and cleanse by His gracious faithfulness and justice!

Reckoning with sin is not taking Christianity too seriously, it is affirming Christianity as true as it reveals ourselves but also reveals the True God!