Committing a sin at first may not be easy, but if one soccumbs to his desires and Satan's temptations, he may reach a point at which he commits sins without the least remorse.
At first, committing a sin may be difficult. Over time, however, committing it becomes easy. Then, one may even enjoy doing it, and if he continues going down this path, that sin becomes a habit. When this happens, darkness engulfs the heart and one would be ready to commit an even graver sin.
In order to not let that happen, one has to stand guard all the time. When Satan or his carnal desires tempt him to commit a sin he should recite the following verse to himself,
قُلْ أَذَلِكَ خَيْرٌ أَمْ جَنَّةُ الْخُلْدِ الَّتِي وُعِدَ الْمُتَّقُونَ كَانَتْ لَهُمْ جَزَاءً وَمَصِيرًا
Say, "Is that better, or the everlasting paradise promised to the God wary, which will be their reward and destination?" (25:15)