I have a question about St. Faustina's prayer: "Eternal Father, I offer you the body and blood, soul and divinity of your most beloved Son and Our Lord Jesus Christ in atonement for our sins and the sins of the whole world."
It does not seem logical for us to offer Jesus' sufferings to the Father – a gift from the Father to us and in this prayer we are returning the same gift to God. It is more proper to offer our sufferings and those of our loved ones to God. But we cannot use others' suffering to free ourselves from the consequence of our wrongdoings.
Most of the time we pay little attention to the wording of a prayer, so you are to be commended for doing so. When saints are canonized, it's not for the content of their visions but for the holy lives they have led. I distinctly remember that this was clarified when Sister Faustina was canonized.
St. Faustina's prayer is unusual in that it seems to assume that we have the power on our own to offer Christ's sacrifice to the Father. Jesus alone can truly offer his sufferings and death, and he has offered them on our behalf. We are powerless to attain salvation or to pray without God taking the initiative.
Jesus hands over to the Father all that he is and has, including our sins which he has taken upon himself. He becomes the one true sin offering. Christ is our Saviour and our intercessor.
However, we, as members of the Body of Christ, share in Jesus' offering and we can offer our own sufferings with his.
You'll notice that in all our liturgical prayers, we pray to the Father through and with Jesus Christ. That is what the Church does through the Mass, that is, it unites the praise and offering of God's people to the perfect sacrifice of praise and offering of Jesus: "Through him (Jesus), then, let us continually offer a sacrifice of praise to God" (Hebrews 13.15).
Look at the Proper of the Mass at the short prayers like the Collect, the Prayer over the Offerings and the Prayer after Communion. These are all prayers for God's assistance through the intercession of Christ. They are addressed to the Father and always end with some version of "through Christ our Lord."
Also go over the various forms of the Canon, that is, the prayers which include the Consecration. There you'll find "Through him, with him and in him (Jesus), O God the Father almighty, all glory and honour is yours, forever and ever."
The second part of your question deals with the forgiveness of sin. There are two steps to this process. First, we have to have a sincere sorrow for having offended God and others, as well as a promise to avoid sinning again.
Second, there is the temporal punishment or restitution that we are obliged to carry out as our sin offends the good of others who are part of the one Body.
In the sacrament of Penance, this is symbolically represented by a penance given by the confessor. But in addition to that, we are obliged to make amends in some way for the wrong we have done to God and our neighbour.
You are right; it is up to each of us and not others to free us from the consequences of our sins. However, others can help us overcome our failings by their prayers and good example.
After all, we are members of one body, the Body of Christ and so we are interested in the welfare of other parts of this Body and are concerned for one another. Therefore, together we unite our prayers and sacrifices to Jesus' suffering and death on the cross for he alone has redeemed us.
As St. Paul said so clearly: "I am now rejoicing in my sufferings for your sake and in my flesh, I am completing what is lacking in Christ's suffering for the sake of his body, that is, the Church" (Colossians 1.24).
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