ROME—A former Vatican altar boy has testified that an older seminarian would come into his bed at night to perform a sexual act on him in the Vatican’s youth seminary, saying his initial shock gave way to resignation because he feared being sent home. Accuser L.G. testified for the first time Wednesday in the Vatican’s criminal courtroom, in the first-ever case to go to trial alleging sexual abuse within the Vatican walls—among the altar boys who serve at papal Masses in St. Peter’s Basilica. The Rev. Gabriele Martinelli is accused of abusing his authority as a more senior seminarian to force L.G. into “carnal acts,” using violence and threats, from 2007-2012. The former rector of the seminary, the Rev. Enrico Radice, is charged with having helped Martinelli avoid investigators by discrediting L.G.’s allegations as baseless. Martinelli has denied he molested L.G., saying the allegations were unfounded, implausible, and the fruit …