To worship God 'in spirit and in truth' is first and foremost a way of saying that we must worship God by means of Christ. In him the reality has dawned and the shadows are being swept away (Hebrews 8:13). Christian worship is new covenant worship; it is gospel-inspired worship; it is Christ-centered worship; it is cross-focused worship.
D. A. CarsonAuthority can mean different things to different people. For example, some document or other may be authoritative for particular group even though it's not reliable. It's just that the group has accepted that document as authoritative for their group. And some documents are truthful and reliable but they are ignored, so they have no authority for that particular group.
D. A. CarsonThe important thing, Jesus is saying (in Matthew 5:33-37), is to tell the truth and keep one's pledges without insisting that a certain form of words must be used if it is to be binding. No oath is necessary for the truthful person... Their word is so reliable that nothing more than a statement is needed from them.
D. A. Carson