(25 Mar. 708–9 Apr. 715)
A Syrian like his predecessor and described by his biographer as ‘exceedingly gentle’, he should probably be identified with the archdeacon Constantine who was one of Pope Agatho's representatives at the Sixth General Council (third council of Constantinople, 680–81). He later served as apocrisarius for Leo II at the imperial court. His reign is reported to have witnessed the extremes of famine and abundance
Early in 709 Constantine had a brush with Felix, newly elected archbishop of Ravenna, whom he consecrated but who, reasserting the autonomy from Rome that Ravenna had briefly (662–82/3) enjoyed, refused to provide the required oath of obedience and other tokens of submission in customary form. In 712, however, when Felix returned to his see from the exile to which Emperor Justinian II had sentenced him after putting out his eyes, he made his peace with the pope and died (723) in communion with Rome. But the centrepiece of Constantine's reign was the year-long journey (Oct. 710–Oct. 711) he made to the east on the express summons of the emperor. It was Justinian's wish to normalize relations with Rome by reaching a mutually satisfactory agreement about the disciplinary and ritual canons which the Quinisext council (692), held at his instigation, had enacted but which Sergius I had refused to endorse because of the anti-western tone of many of them. The visit, which must have seemed ominous in prospect, proved a triumphant success. Accompanied by an impressive retinue, Constantine was, on Justinian's instructions, royally received everywhere. The negotiations at Nicomedia (Izmit) were conducted with consummate skill by his deacon Gregory (soon to be Gregory II), who in response to the emperor's enquiries convincingly explained the Roman objections to a number of the canons; and the pope seems to have finally approved, verbally at any rate, such of them as were not repugnant to western usage. Justinian, who had ceremonially prostrated himself and kissed Constantine's feet on meeting him and received communion and absolution from him, was evidently well satisfied, and published a decree confirming the privileges of the Roman church, including probably its jurisdiction over Ravenna.
Constantine reached Rome, after a journey troubled by illness, on 24 Oct. 711. Soon after his departure in 710 the new exarch, John Rizocopus, for reasons which remain obscure, had brutally executed several of his senior, most valued officials; the news must have reached the pope at Constantinople, but he had not allowed it to interrupt the discussions. A fortnight after the pope's return (4 Nov. 711) Justinian was murdered by mutinous troops. The new accord between Rome and Byzantium was at once shattered, for the new emperor, Philippicus Bardanes (711–13), was a fanatical monothelite who, repudiating the Sixth General Council (680–81), sent the pope an official exposition of his belief in one will in Christ and demanded his adhesion. This Constantine refused, and in the resulting furious reaction in Rome Philippicus was rejected, his name omitted from public documents and from the prayers at mass, and his likeness removed from churches and the coinage. The exarch took steps to enforce the emperor's wishes, and there were bloody battles in the streets of Rome. The pope played a pacific role, sending out bands of priests armed with crosses and gospel books who prevailed on the anti-imperial mobs to withdraw. Fortunately Philippicus was soon overthrown (3 June 713), and his successor Anastasius II (713–15) promptly dispatched to Constantine formal assurances of his orthodoxy and adhesion to the Sixth General Council.