Jacob 2

4 For behold, as yet, ye have been obedient unto the word of the Lord, which I have given unto you.
5 But behold, hearken ye unto me, and know that by the help of the all-powerful Creator of heaven and earth I can tell you concerning your thoughts, how that ye are beginning to labor in sin, which sin appeareth very abominable unto me, yea, and abominable unto God.
6 Yea, it grieveth my soul and causeth me to shrink with shame before the presence of my Maker, that I must testify unto you concerning the wickedness of your hearts.

Even when we live obedient to the commandments of God, we can allow things into our minds and hearts that will lead us away from Him. When Jacob addressed his people, he acknowledged that “as yet” they had been obedient to the word of the Lord. Nevertheless, he was aware of the things they were thinking about, and knew that the wickedness of their hearts would lead them eventually to act on the thoughts. I have heard something before that I will formulate as follows:
Nurture a thought; breed an act.
Continue an act; develop a habit.
Continue a habit; define a character.

The writer of Proverbs put it simply: For as he thinketh in his heart, so is he (23:7). We have to guard our thoughts. It’s hard to fill our minds with things that are good and uplifting in a world saturated with media focused on the base and degrading. The blessings of vigilance are worth the effort – “let virtue garnish thy thoughts unceasingly; then shall thy confidence wax strong in the presence of God; and the doctrine of the priesthood shall distil upon thy soul as the dews from heaven.” (D&C 121:45)