Since their Baptism on Holy Saturday these converts wore white robes. Now in their everyday dress they must go out as witnesses that “Christ is the Truth” (Epistle). God the “Father” bore “witness” to this at Christ’s Baptism by “water.”
God the “Word” became our “Blood” witness on the Cross. God the “Spirit” gave witness when by Him Jesus was conceived in the womb of Mary. Yes, this is the triple “testimony of God” which we, too, must witness unto others—that “Jesus is God” (Epistle).
To the “doubting Thomases” of all future ages, Jesus gave a new proof of His Divinity in today’s Gospel, “written that you may believe...and that believing you may have life.”