'Only Son' in the Bible

Now as He approached the city gate, a dead man was being carried out—the only son of his mother, and she was a widow. And a large crowd from the city was with her [in the funeral procession].

By faith Abraham, when he was tested [that is, as the testing of his faith was still in progress], offered up Isaac, and he who had received the promises [of God] was ready to sacrifice his only son [of promise];