Jesus could and did forgive sins before the cross. This took place in the OT or otherwise known as a time under the old/first covenant. How could he forgive sins without the shedding of blood?
What changed after the cross? (his death and shed blood ushered in the new covenant)
Christ trampled down death by death, among other things sealing the New Covenant with His blood, as promised at the Last Supper, which we partake of in actuality in the Eucharist, which along with Baptism and other mysteries establishes our covenantal relationship with Christ by grafting us onto His body, the Church (however we define that ecclesiologically; I myself, troubled by certain schisms, am highly tolerant of differences of opinion in ecclesiology, but I like ecclesiological models in which Catholicity is defined in relationship to Communion, because of the importance attached to this by the Fathers of the Early Church as expressed in, for example, the Apostles’ Creed, an ancient Baptismal liturgy used in the Western Church that is textually related to the Nicene Creed which defines the essential parameters of Christian doctrine.*
*As a liturgical excursus, since the liturgy is the defining fabric of Christian faith, according to the principle of Lex Orandi, Lex Credendi, even in the context of churches that only follow a pattern of worship rather than written forms:
My one main objection to the Stone-Campbell movement is their “No Creed but Christ” approach, since they get other things so very much right, that being a rejection of denominational separation and an emphasis on the Eucharist. But I feel that the Nicene Creed should be recited at least every Sunday, and the Apostles’ Creed and other texts used in support, for example, the version of Quincunque Vult that lacks the filioque, which I think I will post to my Christian Forums blog as it is obscure, and other creedal hymns, for example, the Latin hymn Te Deum Laudamus, composed by Saints Ambrose and Augustine, the Greek hymn Ho Monogenes, found in the Eastern Orthodox, Armenian Orthodox, Coptic Orthodox and most prominently, the Syriac Orthodox liturgy, composed by St. Severus of Antioch, and the Syriac hymn Haw Nurone, composed by St. Jacob of Sarugh, used in the Syriac Orthodox Church, and one would hope in the pre-Vatican II Maronite and Syriac Catholic Liturgies which were derived from the Syriac Orthodox liturgy, since these two churches separated from it in schism, as it basically is an affirmation of the Real Presence in the Eucharist, and does not contain anything contrary to a Chalcedonian Christology (indeed, I would argue that the Miaphysite and Chalcedonian Christologies are not contradictory, but that is another argument).
Essentially all of these creedal hymns affirm a covenantal relationship which was sealed on the Cross, the Covenant
@ewq1938 refers to, which is approached by us either through Baptism and the Eucharist or alternatively the shedding of our own blood for confessing Christ, since Scripture makes it clear that if we are killed for confessing that Jesus Christ is the Lord, we will be saved. This is often called the “Baptism of blood” when it happens to the unbaptized. Indeed even the Good Thief represents an example of the power of confessing faith in Christ, for he is saved in extremis, and interestingly enough is the first person to be directly saved by Christ as He actualizes the covenant through His death and resurrection, by which He redeems and sanctifies fallen humanity.