Sola Scriptura: If there is any concept that is shown plainly throughout scripture, it is that God is holy, and we are not. The publican’s confession reflects a reliance on the testimony of scripture.
Sola Fide: This confession displays nothing but faith, and total and utter dependence on God to save. There are no corollaries, dependent clauses, or outs in case there is another way for salvation, but only a trust in the mercy of God.
Sola Gratia: “Lord be merciful to me a sinner” is both the confession of David in Psalm 51, and it aptly reflects what the only means is for salvation, the grace of God. The publican’s confession is not “Lord be merciful to me a sinner as I cooperate with the grace that you give to me.”
Solus Christus: This phrase recognizes a dependence upon the Lord for salvation, implicitly Christ. There is no other Lord but Him who the publican cried out to.
Soli Deo Gloria: Since “Lord be merciful to me a sinner” is grace alone, it recognizes the goodness, righteousness, and holiness of God and the wholesale failures of man. The glory goes only to God for His goodness and mercy, while the publican is left humbled in his confession.