According to this site, mantra diksha not taken under a Vaishnava Guru are unqualified in worshipping Lord Vishnu. The site has cited the following verses which are supposedly from the Kartika-Mahatmya of the Skanda Purana (which so far, I have been unable to locate):
"te narah pasavo loke, kim tesam jivane phalam; yair na labdha harer diksa, narcito va janardanah"
English Translation: "Unfortunate people do not accept Mantra Diksha from a bonafide Vaishnava guru in any of these four sampradayas and thus they remain unqualified to worship Lord Krishna. Such people are simply like animals since there is no higher purpose for their human form of life."
"tapasvinah karma nistha, sresthaste vai nara bhuvi; prapta yaistu harer diksa, sarva dukha vimocani"
English Translation: "Those persons who accept Diksha from a bonafide Vaishnava Spiritual Master from any of these four sampradayas are completely delivered from all the miseries of material existence. In this world, they become superior even to the great ascetics and karma-nistha yogis."
Are these verses really present in the Skanda Purana? Are the translations of these verses accurate?