Isaiah 6 depicts the prophet seeing the Lord enthroned while seraphim proclaim YHWH's holiness.
Isaiah 6:1-10; Isaiah 53:1 are the direct backgrounds.
John's antecedent is Jesus: Isaiah saw his glory. This identifies the pre-incarnate glory of Christ with the glory manifested in Isaiah's YHWH vision.
This is one of John's strongest Old Testament divine-identity connections because it explicitly interprets a YHWH theophany Christologically.
Despite many signs, many do not believe in Jesus.