Put briefly, sound doctrine is important because it tells us who Jesus Christ is. Every point of Christian doctrine reflects on the identity of our Divine Lover and to lose Christ's greatest gift to his followers, a share in the life of the Trinity:
Let what you heard from the beginning abide in you. If what you heard from the beginning abides in you, then you will abide in the Son and in the Father. (1 Jn 2:24)
The following excerpt from Thomas Dubay's Authenticity (Ignatius Press, 1997, ch. 8) expresses the same, more explicitly drawing from Sacred Scripture. Particularly pointed are Pastoral Epistles, the letters St. Paul wrote to those he had established in the ministry of teaching and governing the Churches.
For biblical men, authentic teaching was considered more important than miracles in distinguishing the true prophet from the false. The former could be known in that his teachings were in accord with received revelation (Dt 13:1-6) The wrath of Yahweh breaks forth upon the heads of the false prophets who were not sent by him but speak visions of their own imagination. It was promised that in the new dispensation God would place a new spirit, a new power in the hearts of the people, and from this inner source they would obey his outer word (Ezek 36:26f; Jer 31:33). Jesus promised that this inner power, now known to be the Holy Spirit, would teach all that Jesus had taught (Jn 14:26). That man or woman cannot be of the Spirit who contradicts the truth the Lord has already committed to his Church. In this same spirit, St. Paul remarked at mid first century that no one can confess Jesus' lordship correctly without the aid of the Holy Spirit, and conversely no one who is speaking in the Spirit will ever utter a false statement about Jesus (1 Cor 12:3).
But as the first century wore on and false teachers began to proliferate, the New Testament writings became more and more vehement about the critical necessity of sound doctrine. Paul curses those who teach a gospel other than what has been taught (Gal 1:6-9). The Colossians are admonished not to be deceived by any seductive philosophy or mere human opinions (Col 2:7-8).... Paul on the other hand appealed to the traditions handed down by the apostles. So it goes in any age. Those deviating from the gospel speak of their position in attractive, relevant-sounding terms. But Paul was on to this sort of tactic. The author of Hebrews advises his readers not to be carried away by all sorts of strange teaching but rather to imitate the faith of their leaders (Heb 13:7-9).
The pastoral Letters abound in references to sound teaching as a crucial need toward the end of the first century. Timothy is to warn people against teaching false doctrines (1 Tim 1:3), and he is reminded that the Church of the living God is the pillar and safeguard of the truth (1 Tim 3:15). He is to guard the rich deposit of the faith with the aid of the indwelling Holy Spirit (2 Tim 1:14). This faith is to be handed on to trustworthy teachers and not to anyone at all (2 Tim 2:2). Then there follows a series of admonitions to correct error, even to the point of being persecuted, for there will come a day when people will not tolerate sound doctrine but will gather for themselves teachers who will tickle their ears with falsehood.
Titus receives the same message: Hold fast to the authentic tradition, encourage sound doctrine, refute those who contradict it (Titus 1:9; 2:1, 8). This pastoral insistence is of one piece with the rest of the Pauline emphasis on oneness of mind in doctrine and practice. One operates on this ice if he sets the pastoral Letters against the other writings of the Pauline corpus. Raymond Brown notes that "there are first-rate Pauline scholars who still consider Paul as the author of the pastoral letters (int he broad Biblical sense of 'author,' not necessarily in the current, restricted sense of 'writer'). And this is my own view", he adds. Brown remarks that even if further study should one day disclose that the pastorals are pseudepigraphical, their composer must have felt that
their theology was close enough to Paul's to warrant the assumption of the Pauline mantle. A study of pseudepigrapha in the Bible seems to indicate that generally a pseudepigraphical work is attributed to an author because it is a continuation of his thought, style, or spirit, rather than because it is designed to correct his theology. Therefore we must proceed with care in drawing a sharp line of demarcation between Pauline theology and that of the Pastorals. (New Testament Essays, 1965, ch. 3)
[The concern of St Paul expressed in the Pastoral epistles over the rapid proliferation of errors is found] in other late-first-century writings.
Throughout the First Letter of John, we find a repeated concern for the divisions in the Church that are caused by the false teachers. This is so much the case that it has been suggested that the principal theme in the letter is discernment: How can the community members tell the true teachers from the false? This concern is expressed in different ways. The Letter itself is written that the recipients may be in union with those who articulate the community's faith (1:3). If they walk in the light, they are united to one another (1:7). As long as they keep alive what they were taught in the beginning, they share in the trinitarian life of Father and Son (2:24). Of this text Bruce Vawter observes, "The safeguard of the true Christian who would avoid the dire consequences of this false teaching is to hold firmly tothe teaching received through the apostolic preaching" (JBC 62:17, cf. Acts 2:42). The true prophet can be distinguished from the false by his doctrine on the Incarnation (4:2-3, 15; 5:1, 5).
Though it is breif, the Second Letter of John shows the same concern. Noting that there are many "deceivers in the world", the author make the point that only they who keep to what they were taught can have the Father and the Son with the, (2 Jn 9-10). THis sounds harsh to ecumenically tuned ears, but whatever else one may say of it, he must agree that correct doctrine was viewed as immensely important. The Thurd Letter hints at the same theme: the author rejoices to hear that his "children" are living according to the truth (3 Jn 4). It is significant that the Book of Revelation concludes with a solemn warning that to anyone who adds to the prophesies in the book God will add all the plagues mentioned in it, while anyone who cuts away anything will be cut off from the tree of life and the holy city (Rev 22:18-19). The Letter of Jude warned that the false teachers are surely to be punished (Jude 3-19). The same concern occurs in 2 Peter, where the author makes the point that, just as there were false prophets in the old dispensation, so there will be false teachers in the new. They introduce their own disruptive opinions and thus disown the Master and destroy themselves in the process (2 Pet 2:1-2).
.... Doctrinal fidelity as a mark of authenticity is sometimes an obvious sign, sometimes not so obvious. Flat contradition of magisterial teachings is a clear indicator of inauthenticity, for the Holy Spirit does not lead the individual to reject the teaching of the very leaders he himself has established to protect the truth (Acts 20:28). While the genuine theologian attempts to develop and complement and unfold biblical and ecclesial teachings, he does not reject it. Scripture could not be more clear.
But the Counselor, the Holy Spirit, whom the Father will send in my name, he will teach you all things, and bring to your remembrance all that I have said to you.
Therefore I want you to understand that no one speaking by the Spirit of God ever says "Jesus be cursed!" and no one can say "Jesus is Lord" except by the Holy Spirit.
As I urged you when I was going to Macedonia, remain at Ephesus that you may charge certain persons not to teach any different doctrine,
if I am delayed, you may know how one ought to behave in the household of God, which is the church of the living God, the pillar and bulwark of the truth.
guard the truth that has been entrusted to you by the Holy Spirit who dwells within us.
and what you have heard from me before many witnesses entrust to faithful men who will be able to teach others also.
but if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin.
Let what you heard from the beginning abide in you. If what you heard from the beginning abides in you, then you will abide in the Son and in the Father.
And they devoted themselves to the apostles' teaching and fellowship, to the breaking of bread and the prayers.
Any one who goes ahead and does not abide in the doctrine of Christ does not have God; he who abides in the doctrine has both the Father and the Son. If any one comes to you and does not bring this doctrine, do not receive him into the house or give him any greeting;
But false prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them, bringing upon themselves swift destruction. And many will follow their licentiousness, and because of them the way of truth will be reviled.
Take heed to yourselves and to all the flock, in which the Holy Spirit has made you overseers, to care for the church of God which he obtained with the blood of his own Son.
There is nothing so stupendously stupid as the common saying, `Religion can never depend on minute disputes about doctrine.' It is like saying that life can never depend on minute disputes about medicine.
- G.K. Chesterton