Isaiah teaches that sins are forgiven through Christ's blood
Justin: For Isaiah did not send you to a bath, there to wash away murder and other sins, which not even all the water of the sea were sufficient to purge; but, as might have been expected, this was that saving bath of the olden time which followed those who repented, and who no longer were purified by the blood of goats and of sheep, or by the ashes of an heifer, or by the offerings of fine flour, but by faith through the blood of Christ, and through His death, who died for this very reason, as Isaiah himself said, when he spoke thus:
The Lord shall make bare His holy arm in the eyes of all the nations, and all the nations and the ends of the earth shall see the salvation of God. Depart, depart, depart, go out from thence, and touch no unclean thing; go out of the midst of her, be clean that bear the vessels of the Lord, for you go not with haste. For the Lord shall go before you; and the Lord, the God of Israel, shall gather you together. Behold, my servant shall deal prudently; and He shall be exalted, and be greatly glorified. As many were astonished at You, so Your form and Your glory shall be marred more than men. So shall many nations be astonished at Him, and the kings shall shut their mouths; for that which had not been told them concerning Him shall they see, and that which they had not heard shall they consider. Lord, who has believed our report? And to whom is the arm of the Lord revealed? We have announced Him as a child before Him, as a root in a dry ground. He has no form or comeliness, and when we saw Him He had no form or beauty; but His form is dishonoured, and fails more than the sons of men. He is a man in affliction, and acquainted with bearing sickness, because His face has been turned away; He was despised, and we esteemed Him not. He bears our sins, and is distressed for us; and we esteemed Him to be in toil and in affliction, and in evil treatment. But He was wounded for our transgressions, He was bruised for our iniquities; the chastisement of our peace was upon Him. With His stripes we are healed. All we, like sheep, have gone astray. Every man has turned to his own way; and the Lord laid on Him our iniquities, and by reason of His oppression He opens not His mouth. He was brought as a sheep to the slaughter; and as a lamb before her shearer is dumb, so He opens not His mouth. In His humiliation His judgment was taken away. And who shall declare His generation? For His life is taken from the earth. Because of the transgressions of my people He came unto death. And I will give the wicked for His grave, and the rich for His death, because He committed no iniquity, and deceit was not found in His mouth. And the Lord wills to purify Him from affliction. If he has been given for sin, your soul shall see a long-lived seed. And the Lord wills to take His soul away from trouble, to show Him light, and to form Him in understanding, to justify the righteous One who serves many well. And He shall bear our sins; therefore He shall inherit many, and shall divide the spoil of the strong, because His soul was delivered to death; and He was numbered with the transgressors, and He bore the sins of many, and was delivered for their transgression. Sing, O barren, who bearest not; break forth and cry aloud, you who does not travail in pain: for more are the children of the desolate than the children of the married wife. For the Lord said, Enlarge the place of your tent and of your curtains; fix them, spare not, lengthen your cords, and strengthen your stakes; stretch forth to your right and your left; and your seed shall inherit the Gentiles, and you shall make the desolate cities to be inherited. Fear not because you are ashamed, neither be confounded because you have been reproached; for you shall forget everlasting shame, and shall not remember the reproach of your widowhood, because the Lord has made a name for Himself, and He who has redeemed you shall be called through the whole earth the God of Israel. The Lord has called you as a woman forsaken and grieved in spirit, as a woman hated from her youth.'
Source: Dialogue with Trypho (New Advent)