Which verse is cited as affirming the belief that God exists in three persons?

The correct choice highlights Matthew 28, which contains the Great Commission, where Jesus instructs His disciples to baptize in the name of the Father, the Son, and the Holy Spirit. This explicit mention of the three persons is significant because it articulates the triune nature of God—Father, Son, and Holy Spirit—integral to Christian doctrine. The phrasing "in the name of" implies a unity of purpose and action among the three distinct persons of the Trinity, affirming the belief in one God existing in three persons working together in the work of salvation.

The other passages mentioned do not provide a direct reference to the Trinity in this manner. For example, 1 Peter 1 discusses aspects of salvation and sanctification but does not specifically address the triune nature of God. Romans 8 is focused on life in the Spirit and the assurance of salvation, while Acts 2 recounts the coming of the Holy Spirit at Pentecost, which, although pivotal in understanding the Holy Spirit's role, does not explicitly affirm the Trinity in the way Matthew 28 does. Thus, Matthew 28 serves as a foundational text for understanding the concept of the Trinity in the Christian faith.