Should the Bible alone, particularly the New Testament, be the only source of truth in the life of a Christian? The Bible itself does not support this thinking. According to 1 Tim 3:15, the Church is the pillar and foundation of the truth, and the Catholic Church has taught faithfully through the ages that the truth comes from the Bible and Sacred Tradition.
The earliest writings of the New Testament didn’t exist until at least 20 years after the death and resurrection of Jesus. Consider further that the writings weren’t authoritatively proclaimed as inspired and compiled as the New Testament (by the Catholic Church) until an additional three hundred years had elapsed. Until that time, Sacred Tradition was the only basis for the teaching of God’s Word. The Bible complements Sacred Tradition and gives no evidence of replacing it.
The New Testament authors put in writing their personal testimonies while addressing the needs, demands, or emergencies that arose in the early Church. Although they rightfully claimed divine authority (John 13:20), there is no indication that they intended to write a comprehensive system of Christian doctrine. In fact, John 20:30 and 21:25 indicate that not all things Jesus said and did were written down. We know, however, that Jesus still commanded all of these things to be taught (Matt 28:20).
In 2 Tim 2:2, Timothy is told to entrust to reliable men the things he heard Paul say so that others will also be qualified to teach. In 2 Thess 2:15 Paul tells the brothers to hold to the teaching they received whether by letter or word of mouth, and then in 2 Thess 3:6 he commands them to live according to these teachings.
Since the Bible itself sanctions Sacred Tradition and does not claim to replace it, Bible alone thinking must be a tradition of men that is being taught as doctrine in some Christian assemblies. Matt 15:6-9 tells us what Jesus thought of that situation. Wandering from the truth of the Catholic Church is never a good idea. Remain anchored. Remain faithful.