We have been given reason to use and not to use this reason indicates a blockage of consent to learn the Faith.
The Church gives us all we need to work our way through the states of purification, but it is WORK. There is nothing easy about being a Catholic.
But, another sin connected to sloth is pride. Why? Pride keeps one from realizing how much one does not know. A humble person understands that knowledge of self and God means a life-long struggle in suffering and the stripping of self-love. Here is Aquinas on sloth again from this source.
Note that sloth regarding sensuality is mostly a venial sin, but sloth regarding reason is a mortal sin.
Those only seeking consolations need to seriously consider whether their gnostic tendencies point to sloth in rational discourse.
As stated above (I-II, 88, 1,2), mortal sin is so called because it destroys the spiritual life which is the effect ofcharity, whereby God dwells in us. Wherefore any sin which by its very nature is contrary to charity is a mortal sin by reason of its genus. And such is sloth, because the proper effect of charity is joy in God, as stated above (Question 28, Article 1), while sloth is sorrow about spiritual good in as much as it is a Divine good. Therefore sloth is a mortal sin in respect of its genus. But it must be observed with regard to all sins that are mortal in respect of their genus, that they are not mortal, save when they attain to their perfection. Because the consummation of sin is in the consent of reason: for we are speaking now of human sins consisting in human acts, the principle of which is the reason. Wherefore if the sin be a mere beginning of sin in the sensuality alone, without attaining to the consent of reason, it is a venial sin on account of the imperfection of the act. Thus in the genus of adultery, the concupiscence that goes no further than the sensuality is a venial sin, whereas if it reach to the consent of reason, it is a mortal sin. So too, the movement of sloth is sometimes in the sensuality alone, by reason of the opposition of the flesh to the spirit, and then it is a venial sin; whereas sometimes it reaches to the reason, which consents in the dislike, horror and detestation of the Divine good, on account of the flesh utterly prevailing over the spirit. On this case it is evident that sloth is a mortal sin.