1 Ὃ ἦν ἀπʼ ἀρχῆς, ὃ ἀκηκόαμεν, ὃ ἑωράκαμεν τοῖς ὀφθαλμοῖς ἡμῶν, ὃ ἐθεασάμεθα καὶ αἱ χεῖρες ἡμῶν ἐψηλάφησαν περὶ τοῦ λόγου τῆς ζωῆς — 2 καὶ ἡ ζωὴ ἐφανερώθη, καὶ ἑωράκαμεν καὶ μαρτυροῦμεν καὶ ἀπαγγέλλομεν ὑμῖν τὴν ζωὴν τὴν αἰώνιον ἥτις ἦν πρὸς τὸν πατέρα καὶ ἐφανερώθη ἡμῖν — 3 ὃ ἑωράκαμεν καὶ ἀκηκόαμεν, ἀπαγγέλλομεν καὶ ὑμῖν, ἵνα καὶ ὑμεῖς κοινωνίαν ἔχητε μεθʼ ἡμῶν. καὶ ἡ κοινωνία δὲ ἡ ἡμετέρα μετὰ τοῦ πατρὸς καὶ μετὰ τοῦ υἱοῦ αὐτοῦ Ἰησοῦ Χριστοῦ. 4 καὶ ταῦτα γράφομεν ἡμεῖς, ἵνα ἡ χαρὰ ἡμῶν ᾖ πεπληρωμένη. (1 John 1:1-4 NA28)
1 What was from the beginning which we have heard–which we have seen with our eyes, which we beheld and touched with our hands—with respect to the Word of Life—2 indeed the life was manifested and we have seen it and we give testimony and we proclaim to you the eternal life which was with the Father and was manifested to us—3 what we have seen and we have heard we proclaim to you also that you may also have fellowship with us; and indeed our fellowship is with the Father and with his son Jesus Christ. 4 These things we write that our joy maybe made full. (1 John 1:1-4 translated from the NA28 Greek text)
Eusebius was a contemporary of Constantine. His book, The Church History, covers the time of the Apostles and spread of Christianity over the Roman world during and after their time. He brings up an interesting point about a third of the way through the book in that while the Apostles were still alive as well as those who were eye witnesses to their lives and ministries and that included those who had actually seen our Lord Jesus before his crucifixion and after his resurrection, our enemy could only successfully attack with limited success from within the Church. Heresies were easily dealt with as John did in the passage above in dealing a deathblow to the major doctrines of an early form of Gnosticism. However, after these first generation leaders of the Church died or were martyred (which was more likely), our enemy changed tactics and so began the era of wolves coming into the Church to lead astray the sheep with deadly heresies such as Arianism. What is the solution? What has always been the solution? It is the same as what the Apostle John did in 1 John and the Apostle Paul did in 1, 2 Timothy, Titus, and 1 Corinthians and the Apostle Peter in 2 Peter. They commanded leaders to teach and preach the Word.