Prior to the Council of Nicea, every congregation held to the Rule of Faith, which contained the essentials of Christianity. These were apparently memorized at baptism by all new Christians. It was sometimes referred to as "the Rule" and "the Rule of Truth."
"Let us give up purposeless and fruitless cares and approach the holy and venerable Rule of our calling. Let us attend to what is good, pleasing, and acceptable in the sight of Him who formed us. Let us look steadfastly to the blood of Christ and see how precious that blood is to God, which has set the grace of repentance before the whole world. Let us turn to every age that has passed and learn that the Lord has granted a place of repentance to all that would be converted to Him. Noah preached repentance, and as many as listened to him were saved." –Clement of Rome
"The one who retains the Rule of the Truth received through baptism unchangeable in his heart will surely recognize the names, the expressions, and the parables taken from the Scriptures [by the Gnostics] but will by no means acknowledge the blasphemous use which these men make of them." –Irenaeus
What exactly is the “Rule of Faith” mentioned repeatedly by the early Christians?
"The rule of faith, indeed, is altogether one, alone immoveable and unchangeable. The rule is: To believe in the only one God Almighty, the Creator of the universe, and His Son Jesus Christ, born of the virgin Mary, crucified under Pontius Pilate, raised again the third day from the dead, received in the heavens, sitting now at the right hand of the Father, destined to come to judge the living and the dead through the resurrection of the flesh." –Tertullian
In other words, the Rule of Faith is the Essentials of the Faith:
"For I delivered to you as of first importance what I also received, that Christ died for our sins according to the Scriptures, and that He was buried, and that He was raised on the third day according to the Scriptures." 1 Corinthians 15:3-4