"Perhaps the Lord needs such men on the outside of His Church to help it along... They are among its auxiliaries, and can do more good for the cause where the Lord has placed them, than anywhere else. … Hence, some are drawn into the fold and receive a testimony of the truth; while others remain unconverted … the beauties and glories of the gospel being veiled temporarily from their view, for a wise purpose. The Lord will open their eyes in His own due time. God is using more than one people for the accomplishment of His great and marvelous work. The Latter-day Saints cannot do it all. It is too vast, too arduous for any one people."

Elder Orson F. Whitney of the Quorum of the Twelve

Quoted by President Ezra Taft Benson in 1972:
https://www.lds.org/ensign/1972/07/civic-standards-for-the-faithful-saints?lang=eng

And more recently here on the church interfaith page:
http://www.mormonnewsroom.org/article/interfaith