Joseph Smith
"Respecting the Melchizedek Priesthood, the sectarians never professed to have it; consequently they never could save any one, and would all be damned together. There was an Episcopal priest who said he had the priesthood of Aaron, but had not the priesthood of Melchizedek: and I bear testimony that I never have found the man who claimed the Priesthood of Melchizedek. The power of the Melchizedek Priesthood is to have the power of 'endless lives'; for the everlasting covenant cannot be broken." (Teachings of the Prophet Joseph Smith, p. 322)
Joseph Smith
"Many objections are urged against the Latter-day Saints for not admitting the validity of sectarian baptism, and for withholding fellowship from sectarian churches. Yet to do otherwise would be like putting new wine into old bottles, and putting old wine into new bottles. What! new revelations in the old churches? New revelations would knock out the bottom of their bottomless pit. New wine into old bottles! The bottles burst and the wine runs out! What! Sadducees in the new church! Old wine in new leathern bottles will leak through the pores and escape. So the Sadducee saints mock at authority, kick out of the traces, and run to the mountains of perdition, leaving the long echo of their braying behind them." (Teachings of the Prophet Joseph Smith, p. 192)