Will Allah forgive whoever concealed the truth of adoption from a child until he reached twenty years of age?

Adoption is forbidden in Islam. It involves attributing a child, whether of unknown or known lineage, to a man, based on the Almighty's saying: "Call them by [the names of] their fathers; it is more just in the sight of Allah." Attributing oneself to other than one's father is a major sin, as stated in the Hadith (definition): "Whoever attributes himself to other than his father… upon him is the curse of Allah." There is no blame on someone who adopted out of ignorance. As for one who did so knowingly and then repented, repentance nullifies what came before it.