Hey Reccanboy, you said:
It would've made sense to me if they were solely the merits of Christ, but what basis is there to believe that the excess merits of Mary and the Saints can be applied to us, especially centuries after their deaths?
Well, for starters, the few passages below (there are more) - remind us that those who have passed on are not dead; just their mortal bodies; their eternal soul lives on...
Just as nothing is lacking regarding Jesus' afflictions, (His atoning sacrifice) - also nothing is lacking regarding Jesus' infinite merit; nothing (no "excess merits") - from mere mortals is required, but nonetheless, Jesus still asks us to pray and intercede for others, as per scripture. If all that is required is Jesus' infinite merit, drawn from His death, on the cross, for the Christians redemption/salvation, which of course is true, then why the need for prayer, on our part? The answer can be found in sacred scripture; it's because we are co-heirs. Scripture tells us that if we are God's children, then "we are heirs—heirs of God and co-heirs with Christ, if indeed we share in his sufferings in order that we may also share in his glory." Just as we are heirs—heirs of God and co-heirs with Christ, by sharing in His sufferings, we are also co-heirs with Christ by virtue of our prayers; it's a participatory action. Is it necessary? no. Is it part of Jesus' plan, as per scripture? yes! In essence, Jesus expects us, as His followers, to bear a sense of the burden and responsibility, which is why Jesus tells us to "pick up our cross" and urges us, via scripture, "that supplications, prayers, intercessions, and thanksgivings be made for all people." Again, why, if the infinite merit of the cross is all that is necessary, which of course is true? Logically, it is because meritorious prayer and intercession on the part of the Christian, "made for all people," draw said people closer to the infinite merits of Christ, who is the way and the truth and the life, and the only way to the Father. All earthly or heavenly prayers, ultimately, coalesce and converge on Jesus, where salvation is found.
Our meritorious prayers, be it from earth or heaven, do not devalue or add to the infinite merits of Christ, nor are they necessary, just as Paul's sufferings did not devalue or add anything to Jesus' infinite sacrifice of atonement, nor were his sufferings necessary, but, according to scripture, both are part of Jesus' grand plan for the salvation of lost souls. As Christians, conformed to Christ, (imperfectly here on earth, and perfectly, in Heaven) - our prayers and intercession are required petitions, (as per scripture) - to God and therefore are a required participation, joined to the One and only infinite sacrifice that merits eternal life for souls that were cut off from God's glory. They are absolutely not an addition to the infinite merits of Christ.
Just as we are suppose to participate in redemptive suffering, by picking up our crosses and following Jesus, we are also to participate in Jesus' salvific plan for souls, with our prayers and intercessions, which of course, includes those prayers and intercessions of the saints, in heaven, as per the passages below. Our prayers absolutely matter, according to scripture, and how much more efficacious our prayers and intercession must be once we are, in fact, perfectly united to God, before the heavenly throne? And it's those heavenly prayers and intercession of the saints in heaven, offered up before the throne of God, that comprise the treasury of merit, that in no way add to the infinite merits of Christ.
For example, scripture tells us that worship and the prayers of the saints in heaven, (along with the voice of the many angels, numbering thousands upon thousands, and ten thousand times ten thousand) - are offered up before the Throne of God and the Lamb. The heavenly saints *("cloud of witnesses" - the spiritual aggregation of Old Testament saints as well, who hoped for their reward in Christ encircling the throne) *- are forever before the throne of God offering of their prayers. How cool would that be? The golden vials full of incense, which are the prayers of saints in heaven, represent the intercessions of the glorified souls in heaven. The souls/saints in Heaven still continue to pray just as they did here on earth, only their prayers are far more efficacious for they are now perfectly united to God. These saints have finally passed into the inner circle and have drawn near to the Throne of God, forever gazing upon the golden vials full of incense, which represent the prayers of the saints, again, which comprise the treasury of merit, which is just a term, like the Trinity, the Catholic church used to define the doctrine:
Then I saw a Lamb, looking as if it had been slain, standing at the center of the throne, encircled by the four living creatures and the elders. The Lamb had seven horns and seven eyes, which are the seven spirits[a] of God sent out into all the earth. He went and took the scroll from the right hand of him who sat on the throne. And when he had taken it, the four living creatures and the twenty-four elders fell down before the Lamb. Each one had a harp and they were holding golden bowls full of incense, which are the prayers of God’s people. Rev. 5
Another angel, who had a golden censer, came and stood at the altar. He was given much incense to offer, with the prayers of all the saints, on the golden altar before the throne. Rev. 8
The questions remain:
Why are the saints in heaven praying?
For whom are these prayers offered?
How do we draw from this heavenly treasury of prayer of the saints in heaven?
The Catechism of the Catholic Church has the answer: