According to St Jerome and Origen, Joseph had no suspicion of adultery because he knew the modesty and chastity of Mary. Moreover, he had read in Scripture that the virgin would conceive and that “a shoot shall sprout from the stock of Jesse, and from his roots a bud shall blossom.” He knew also that Mary was descended from the line of David. Thus it was easier for him to believe that Isaiah’s prophecy had been accomplished in her than to think that she could have let herself descend into debauchery. This is why, considering himself unworthy to live with a person of such great sanctity, he wanted to send her away secretly – like when Peter says to Jesus, “Depart from me, Lord, for I am a sinful man!”
– St Thomas Aquinas
MEMORARE TO ST JOSEPH
Remember, O most pure Spouse of the Virgin Mary, St Joseph my beloved patron, that never has it been heard that anyone invoked thy patronage and sought thine aid without being comforted. Inspired by this confidence, I come to thee and fervently commend myself to thee. Ah, despise not my petition, dear Foster Father of our Redeemer, but accept it graciously. Amen.