Jesus specifically promised that the Holy Spirit would guide the apostles into all truth (John 16:13; 15:26-27; 14:26). If you believe that Jesus is the Son of God, then you must believe that He kept that promise, and that the New Testament contains all truth (2 Peter 1:3; Jude 3). 2. The New Testament contains God’s revelation for the New Covenant (Jeremiah 31:31-34; Hebrews 8:86ff), but nowhere is there a promise of another covenant to come. 3. In the Bible the priesthood of a covenant and the law of that covenant either stand or fall together (Hebrews 7:12). Seeing that Jesus holds His priesthood forever (Hebrews 7:24), that in turn means that the New Testament is the final revelation from God to mankind.