Punctuation is more or less an educated guess1, but most modern critical texts (including Byzantine critical texts which are sympathetic to the TR) render this as a question (Ἄρτι πιστεύετε;). As a question, Jesus' words cast doubt on his disciples' faith, which the 1984 NIV translation committee may have wished to avoid. Translating this as a statement, while a minority position on the translation of this text, is not wrong. Most English translators (from the KJV through most modern translations) render this as a question (as do the critical Greek texts).2
1 cf. http://www.friktech.com/rel/canon/types.htm
2 St. Augustine, writing in the late fourth or early fifth century, also appears to read this verse as a question. I do not currently have access to a Greek manuscript for this tractate, so this is based solely on the English translation of the tractate (hence why this is only a footnote for further information).
Augustine of Hippo, "Lectures or Tractates on the Gospel According to St. John", trans. John Gibb and James Innes, in A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, First Series, Volume VII: St. Augustin: Homilies on the Gospel of John, Homilies on the First Epistle of John, Soliloquies, ed. Philip Schaff (New York: Christian Literature Company, 1888), 392. This can be read online for free.