After Piccolomini left Scotland, he returned to Basel in 1436. Though still a layman, Piccolomini soon managed to gain a certain esteem in connection with the council. His cleverness and rhetorical talent procured him the post of abbreviator, and caused him to be commissioned on various embassies.
But when it was proposed to nominate him as conclavist in behalf of electing a successor to Eugenius IV., whom the council had pronounced to be deposed, he declined this honor, as he wished to avoid consecration in order that he might still indulge in pleasures not permitted to the clergy.
In the year 1438 or 1439, Piccolomini began his Commentarii on the Council of Basel; in 1440, he wrote the Libellus dialogorum de auctoritate consilii generalis.
Wide prospects were disclosed to him when, in 1442, he attended the imperial diet at Frankfort as envoy. It was there that the bishops of Chiemsee and Treves recommended him to King Frederick III., who crowned him with the laurel, poet of scandalous verses though he was; and then took him into his own service as secretary.
An index to his mood and frame of mind at that time is found in a letter addressed to his father from Vienna, Sept. 22, 1443. He asks him to receive in his home one of his own (Piccolomini's) illegitimate sons; and adds by way of excuse, that he, "of course, was no capon, nor did he belong to your cold natures," casting at his father the shameless comparison: "You know what sort of a chanticleer you were yourself."
If, therefore, a "conversion" of Piccolomini is supposed to have occurred in the following year still this hindered him not from publishing so lascivious a tale as "Euryalus and Lucretia"; and the play Chrysis. And if he writes under date of Mar. 6, 1446: " I am a subdeacon; something I once thoroughly abhorred to be. Levity has left me," the latter acknowledgment need not be taken for very serious repentance. The mainspring rather appears in what he writes two days later: " I own to you, dearest brother, I am satiated, surfeited; I have grown disgusted with Venus . . . Venus even shuns me more than I abominate her."
Simultaneously with his "conversion," as secretary of Frederick III. he changed the direction of his ecclesiastical statecraft. While Felix V. and the Council of Basel still regarded him as the advocate of their interests, he posed even in Vienna as one of the " neutrals," and as such openly appeared at the Nuremberg diet of 1444. The resolution passed by this diet, that the status of " neutrality " should last till 1445, but that Pope Eugenius IV. should then be requested to convoke a new council, was conveyed to Rome by Piccolomini in person; and if, indeed, he did not there contrive to gain approval for his errand, he still gained the entire favor and pardon of Eugenius IV. as far as his own course was concerned. At the diet of Frankfurt (September 1446) Aeneas was instrumental in changing the majority of the electors from their hostile position towards pope and emperor into a friendly one. He brought the good news to Eugenius shortly before his death (February 7, 1447), but before his death was made his papal secretary. He made friends with the new pope, Nicholas V, by whom he was made Bishop of Trieste in 1447 and then of Siena in 1450. He was an agent of Frederick in making the celebrated concordat of Vienna (also called concordat of Aschaffenburg) in February 1448. His services to pope and emperor brought him the titles of prince of the empire and cardinal, positions which he used rather unscrupulously to get many lucrative benefices into his hands . Those in Germany brought him two thousand ducats a year.
All that he formerly assailed at Basel, and what he wrote to the praise of the council, he retracted by appeal to Augustine in the bull In minoribus of Apr. 26, 1463. Even previously, in the Epistola retractationis (cf. F. H. Reuech, Der Index der verbotenen Bucher, i. 40, Bonn, 1883), he had expressed himself in similar terms. Pius felt how much the position of the papacy had fallen in importance since the days of Urban and Innocent III, and, believing that the change was due to the general councils which had asserted power over the popes, he changed his position, which before his election to the papal throne had been that of a warm advocate of the conciliar claims, and issued (January 1460) the bull Execrabilis et in pristinis temporibus inauditus, in which he condemned as heretical the doctrine that the councils were superior to the popes, and proclaimed the anathema against anyone who should dare to appeal to one.
And as touching his Commentarii on the Council of Basel, which during the sixteenth century found their way to the Index, he offset the same, in the years 1448-51, with a work advocating the papal point of view.
Again, with reference to his obscene writings, about the period of 1440, the pope exclaims to his readers: "Away with that AEneas, and now receive Pius!"