The "word of Christ"--at least for us today--can only be the Holy Scriptures, so this is an exhortation to memorize Scripture. (Psalm 119:11). Note that Christ and the apostles frequently quoted from memory appropriate Scriptures, sometimes lengthy passages, in their discourses and sermons. This is the only occurrence of the phrase "word of Christ."

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8