The phrase "worship in spirit and truth" comes from John 4:6-30, during Jesus' conversation with the Samaritan woman at the well. It was well known that Jews worshipped God at Jerusalem and Samaritans worshipped Him at Mount Gerizim. The woman asked Jesus which was the right place to worship, and He responded by telling her that a time was coming very soon when true worshipers would not worship according to a location, but they would worship the Father "in spirit and truth." He was implying that under the soon-to-be established New Covenant in Jesus' blood, location would no longer be an issue because the Holy Spirit would be with every believer, no matter where they were.
What does it mean to worship the Lord in spirit and truth?
The New Covenant, in which salvation, atonement for sin, and intimacy with God would depend on faith in Christ, replaced the Old Covenant (which was the Mosaic Covenant). The Mosaic Law dictated specific times, places, and ways that were deemed appropriate for worship. When those ways were violated, it was very serious, and could even result in death (Leviticus 10:1-2). But Jesus ushered in the New Covenant, in which man would no longer be responsible to carry out the Mosaic Law in order to be pure before God. Instead, the purity of Jesus could be applied, through faith, to all who would believe (John 1:12-13; Romans 4:3-5). Under the New Covenant, specific locations would not be necessary—a person would only need to worship in truth, and in spirit.
What does it mean to worship in spirit and in truth? True worship is something that flows from the human heart (or the human spirit) as a result of revelation (John 4:13-14). Truth is revealed to the heart by the Holy Spirit and the Word of God (John 14:15-17, 25-26; John 16:13-15; 2 Timothy 3:16-17; 2 Peter 1:20-21). Isaiah prophesied about God's chosen servant, who was Jesus Christ, calling Him a "covenant for the people, a light for the nations, to open the eyes that are blind" (Isaiah 42:6). Just as Jesus chose to reveal Himself as the Messiah to the woman at the well (John 4:10, 25-26), God reveals Himself to each of us in His time. The result of seeing God clearly is worship, because He is glorious. Oftentimes, men offer worship to God out of a sense of duty, or in expectation of blessing, or as a result of being taught a tradition. None of these types of worship are done in spirit and in truth. They are done for human ends, by human means. True worship is given from the human spirit by the power of the Holy Spirit and in the knowledge of the truth.
How can I worship the Lord in spirit and truth (John 4:23-24)? What is true worship?
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Is there a biblical theology of worship?
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Truth about the Christian Life