In a brief letter, Saint John Paul II considers NFP in its indispensable context of the Church’s teaching on the truth of the act of love. Here “spouses become sharers in God’s creative action,” never putting aside the sacred character of their vocation.
Read this brief, lucid letter here:
http://www.usccb.org/issues-and-action/marriage-and-family/natural-family-planning/awareness-week/upload/Why-NFP-Differs-from-Contraception-JPII.pdf