Sentence 19
And therefore it was imputed to him for righteousness. - Romans 4:22
Abraham believed God. He was fully persuaded. Therefore, God imputed it to Abraham for righteousness. God did not remember that Abraham, at one time, was not fully persuaded. This example of Abraham shows God not imputing sin, but, rather, imputing righteousness.
Sentence 20
Now it was not written for his sake alone, that it was imputed to him; But for us also, to whom it shall be imputed, if we believe on him that raised up Jesus our Lord from the dead; Who was delivered for our offences, and was raised again for our justification. - Romans 4:23-25
Now it was not written for his sake alone, - That God imputed righteousness to Abraham was not written only for Abraham's benefit, but for all those who believed God like Abraham.
that it was imputed to him; - That righteousness was imputed to Abraham.
But for us also, to whom it shall be imputed, - It was also written for the benefit of those to whom righteousness shall be imputed
if we believe on him that raised up Jesus our Lord from the dead; - The condition of receiving imputed righteousness. The condition is that we believe God who raised Jesus our Lord from the dead.
Who was delivered for our offences, - Jesus was sent to earth and then to the cross for our offences. This is a succinct statement of the substitutionary death of our Lord for our sins.
and was raised again for our justification. - The reason that God raised Jesus from the dead was so that people could be made just in the sight of God. Jesus' sacrifice permanently satisfied the LORD God's requirements, and that fact is declared by the Lord Jesus Christ's resurrection from the dead. The resurrection is how we know that the the requirements were satisfied.