Jesus repeatedly warned his followers that they would not know when he would return. He was not warning unbelievers, he was warning them!!
Why was he warning them? Because of the unsavoury elements which would be present amongst his followers, prior to his return.
2 Thess. 2:4, 1st Peter 4:17, and Revelation chapters 2 & 3 (and other texts) confirm and justify Jesus' warnings to all who claim to be Christians. The following illustrations given by Christ, should be a wake up call to all Christians as to the seriousness of the condition they would in, whether they realised it or not!
(Matthew 24:45-48) Who really is the faithful and discreet slave whom his master appointed over his domestics, to give them their food at the proper time? Happy is that slave if his master on arriving finds him doing so. Truly I say to YOU, He will appoint him over all his belongings. "But if ever that evil slave should say in his heart, ‘My master is delaying,’. (Both the faithful slaves and the evil slaves were in the same house!!)
(Matthew 25:14-15) "For it is just as when a man, about to travel abroad, summoned slaves of his and committed to them his belongings. And to one he gave five talents, to another two, to still another one, to each one according to his own ability, and he went abroad. (All these slaves were in the same household!!)
(Matthew 25:33) And he will put the sheep on his right hand, but the goats on his left. (The goats had infiltrated the flock of sheep!!)
(Matthew 13:25) While men were sleeping, his enemy came and oversowed weeds in among the wheat, and left. (The weeds had been planted in the same field as the wheat!!)