Read Grant. Lol. Rundown of the facts. Constantine called the council to quell arguments within the Empire's biggest religion. He couldn't risk civil unrest that could put the empire in further straits than it was already in. He was holding an empire together by threads. He had to keep the military happy and the people from having any reason to rebel against him or put him down like so many Emperors before him.
That said, attendance was required and enforced - at least in the east. Of some 2000 bishops said to attend, only about 6 of them arrived from the west. The term "pope" didn't as yet exist as an office. Nor did the Bishop of Rome have any part in the calling of this council.. it was compelled by the Emperor. And no one was allowed to leave until matters were settled. Now, it's said that other matters were discussed. It's difficult to imagine getting that many people together and not having that happen. It rather seems unlikely you could shut them up. The issue they were called to address was the Arian issue over the nature of Christ vs that of God. Now, The majority liked pretty strict language that would box Arius in, while Constantine suggested looser language that tended to give Arius breathing room. Arius, it must be said, was pals with the Emperor; but was head strong and set on his postion. The double tongued nature of the loose language Constantine suggested only ticked off Arius. And when he blew up, the Emperor banished him.
The fun thing in all this that nobody wants too much light on is that Eusebius was an Arian, as were all of Constantine's closest advisors. And Constantine far and away preferred the Arians. Arius wasn't banished for heresy. He was banished for causing disunity in the Empire. Whether he was right, wrong or indifferent didn't much enter into things, just as it hadn't with the Donatists.
You should note that the Big crime of the Donatists is largely bunk. The truth paints a somewhat less attractive story. Under prior persecution Christian sects were ordered by the emperor to turn over their religious texts to the empire and to honor the pagan gods of Rome like any other 'good citizen'. Many of the sects did so. Donatus was at the head of the line to both decline to follow the emperor's directive and to call it for the sin it was amongst the others. This earned him the indignation of the majority and though he was right, he had to be silenced.
Constantine is credited as having put down Donatism; but, in truth it outlived him. Fact vs. fantasy. Donatus pointed out probably more than that. The roman religions didn't disappear or lose favor, they got a paint job and new names. Theodosius Made Catholicism an official religion and overnight, seemingly, the pantheon of gods for everything under the sun gave way to a pantheon of saints for everything under the sun. Instead of asking the god of dog bites for assistance, you instead would pray to the saint for dog bites. It didn't bother them that crafting idols was against the ten commandments. Nor was it a hindrance that according to scripture, dead people can't have anything to do with the goings on of the living. Regardless of whether they died righteous or evil.. a way had to be made to "keep unity" of the empire which meant inclusion. Thus the modern approach whereby Catholics find "objective truth" in all religions for sake of argument. The roman rites didn't dissapear, they just put on a new dress and pretended to be Christian. It kept the pagans happy with the new state religion that was all the rage. It kept some semblance of unity even if it had to be a lie to happen. And it ultimately led to Rome's bishop and followers defrauding the empire in Constantine's name later - thus the need for the fantasy side of the stories. In their lust for power, and piles of fruadulent documents to bolster their false claims, they goofed and included a fraudulent copy of the documents from nicaea. The Orthodoxers had the originals and rightly corrected the record. The fraud is web posted; but, nicaea has been fraudulently altered in many ways in order to paint a picture that simply was not so. I wouldn't limit my reading on the subject as I know of no single work that deals only with that issue personally. I also don't recommend not reading the Catholic versions. It's actually a good idea to read it and forget it, check your facts, then go back and read their version and compare. It's an eye opener.