What would happen if you took the new testament and nothing else for your source of knowledge? If you spoke where the bible spoke and remained silent where the bible was silent (1 Pet 4:11): You would obeyed the same gospel as the apostles taught (Rom 2:16 f; Gal 1:6-9). You would assemble each first day of the week (Acts 20:7). You would partake of the Lord’s supper and give of your means each Lord’s Day (Sunday) (1 Cor 11:24-26 f; 1 Cor 16:1-3). You would sing without the use of instrumental music (Eph 5:19). Women would remain silent in the assembly (1 Cor 14:33-40). Only faithful Christian men would be preachers, teachers, song leaders, and leaders of prayer (2 Tim 2:2). Qualified elders and deacons would be appointed in every church (Acts 14:23 f; 1 Tim 3:1-13). The church would bear the name of the one who built it (Mt 16:18 ff; Col 1:18; Rom 16:16).
This is exactly what the new testament advocates and teaches. If you do any more or any less, you are not taking the bible as your only source of knowledge. To do more or less is to obey the commandments and doctrines of men which are forbidden in the new testament (Mt 15:7-9). It’s your choice: Do it God’s way and be rewarded, or do it your own way and be condemned (Mt 7:21-23). It can’t be made more simple than that!