I would like to add to what you are saying, and possibly answer the question.
In Gal. 3:16 Paul writes: "
Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ." This tells me that Paul implies Abraham understood God to mean that Christ was Abraham's seed, in the promise He gave him in Gen. 15:5-6, where it says "
Abraham believed God, and it was reckoned to him as righteousness." This implies that all OT saints believed in the future Messiah, as I also think Heb. 11 testifies. Therefore, it is not generic faith that justified them, but faith
in Christ, even though they did not yet know Him.
This would be the same for the tax collector. I get that Jesus is addressing attitude in this passage to show the Pharisees that their self-righteous attitude doesn't justify them. Jesus simply isn't telling the whole story here, namely
why the tax collector:
1. Sees his own sin as horribly sinful
2. Grieves over his sin
3. Humbles himself before God
4. Hopes that God will be merciful to him
5. Believes that God heard his prayer (with hope in a Messiah he doesn't yet know - this is how he could believe that God heard)
6.
Most likely has been satisfied in believing God's forgiveness by receiving a personal response from God such as an affirmation
Although #6 is not necessary for a forensic justification, it would be necessary for the
experience of it. It would depend on whether Jesus meant that he went home justified without knowing it, or justified having known it.
TD