“Blessed Saint Thomas, who are blessed because, seeing the risen Christ and handling His wounds you believed in Him: pray to Him that I, seeing His body and the blood of His wounds each day may also believe Him, and be filled with His love. And may the image of the five wounds go with me wherever I go; and may the blood from them purify me utterly so that every earthly fear, desire or temptation may be driven out of my heart, and so that I may be wholly filled with God’s love and become His servant and the fellow citizen of the Saints. Amen.” 
Here is the first of Merton’s many longer invocations to the saints. He had just completed reading about Thomas in John’s gospel, and he wrote a reflection on what he had read, ending with this prayer. He asks Thomas to pray to God that he might have the same depth of devotion that Thomas had—a devotion that eliminates every evil and instills every virtue. I cannot think of a better request than that.
Making the request to St.Thomas reveals Merton’s belief in the great cloud of witnesses (Hebrews 12:1) as an active presence, not merely a theological statement. To those of us who are not Roman Catholic, we would do well to see the interactive nature between the Church visible (us) and the Church invisible (the cloud of witnesses). The writer of Hebrews saw it, and Merton did too.
And I must go on to say that I do not find asking a member of the Church invisible (one of the saints) to pray for me to be any less appropriate than asking a member of the Church visible (e.g. a friend or pastor) to do so. Intercession is mystery anyway, and perhaps it can take place in heaven as much as on earth.
 Thomas Merton, ‘Run to the Mountain: The Story of Vocation’ (HarperCollins, 1995). By referencing the date, you can find the prayer in any version of the journal you have.