Both Sisinnius and Constantinus were born in the Caliphates of what is now Syria. They are assumed to be actual brothers. Their father was John. Sisinnius was the older by a number of years, born around 650.
When Pope John VII died in late 707, Sisinnius was elected by the nobles and clerics and was consecrated within three months, January 15, 708. He would have made a great pope. He had an outstanding, strong moral character and had pastoral sensibility, caring for all. Unfortunately, at the age of around 55, Sisinnius had a severe, crippling form of gout, rendering him incapable of doing basic care of himself. He needed someone to feed him.
Threatened by the Lombards, the Muslims and the Exarch of Ravenna, Sisinnius was most concerned about the city walls, which were in grave disrepair. Intermittent attacks from each of those enemies over time, had worn down Rome’s infrastructure. His order to repair the walls was eventually acted on about ten years later.
The only pastoral event of note for this poor man was consecration of a bishop from Corsica.
There is only so much a man can do in twenty days. Sisinnius died February 4 of the same year and was buried in the old St. Peter’s basilica.
Of note, this same year, the famous Mont St. Michel, abbey and fortress, was first dedicated in France.
In a month and a half, March 25, Constantine was elected to replace Sisinnius. This man, Syrian by birth, is assumed to be the biological brother of the immediate past pope. He was born in 664, making him considerably younger than Sisinnius. Constantine, the only pope to take on this very Eastern name, was fluent in Greek and immersed in the Eastern rituals and practices. His heart appeared to appreciate the Byzantine culture.
Constantine was one of the Roman legates to the Third Council of Constantinople in 680, as a very young man. He was also one of the legates to deliver a letter of Pope Leo II to Emperor Constantine IV in 682. During that time, he met and associated with the young Prince Justinian. They struck up a rapport at that time.
The major problem the new pope had was that he did not accept the Trullian canons of the Quinsext Council, the 5th and 6th councils held in Constantinople. These addressed questions of marriage among clergy and other disciplinary items.
Early in his papacy, news came to the pope that Archbishop Felix of Ravenna, not a supporter of the pope, was plotting to overthrow the emperor. Emperor Justinian, who had recently risen to the throne again, after 10 years in exile, ordered the archbishop to be blinded. This may have served as a warning to the new pope. It did serve to improve papal-imperial rapport.
By 710, Justinian demanded that the pope appear before him at Constantinople. Pope Constantine went, and was the last pope to visit that city until Pope Paul VI did, over a millennia later. Different from most of the Roman popes, Constantine took the attitude that he would make no delays or excuses to the emperor. His goal seems to have been forestalling further rift between papacy and emperor. The pope left on October 5, 710. After months of travel, he arrived with thirteen companions, eleven who were from the East plus a deacon who became Pope Gregory II and a Latin subdeacon, Julian. Officials who were left behind apparently did not support Pope Constantine’s new rapproachment with the East. Most of the negotiations were conducted by the deacon and some compromises were accepted. The main participants skirted several dicey objections to dogmatic questions in order to produce a political united front. The papal party left for Rome in October, 711.
Less than a month later, Emperor Justinian was assassinated and the new emperor, Philippikos Bardanes, declared his intention to support Monothelism, the belief that Christ had only one nature. Pope Constantine refused to go back to that argument. He would not accept an imperial portrait, nor coins with the emperor’s image. This led the exarch to try to force the issue and clashes began. Constantine was able to keep things under control. Philippikos was overthrown in June 713.
The next emperor, Anastasius II, sent a letter to Pope Constantine supporting the orthodox councils.
The last year of his life was quieter than the previous few. Constantine died in April, 715.