In the early days of computer programming, there was a popular phrase, “Garbage in, garbage out”—meaning, if the programming was lousy, the computer’s output would be useless.

The human mind is a million times more sophisticated and powerful than a computer, but the same principle applies: The quality of our mental output depends on what we put into our minds! If we fill our minds with garbage, then that’s what we’ll spew out.

“For as he thinketh in his heart, so is he” (Proverbs 23:7). That’s one of the most profound proverbs in the Bible. As we think in our hearts, so are we!

Jesus expounded on this truth in Matthew 15: “Do not ye yet understand, that whatsoever entereth in at the mouth goeth into the belly, and is cast out into the draught?” (verse 17). What we put into our bodies eventually comes out—that’s what He was getting at here.

He continued, “But those things which proceed out of the mouth come forth from the heart; and they defile the man. For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies” (verses 18-19). Filling our minds with garbage adversely impacts our overall character—our mannerisms, personality, the way we react and behave.

Jesus also said that every tree is known by the fruit it produces (Matthew 12:33). “O generation of vipers, how can ye, being evil, speak good things? for out of the abundance of the heart the mouth speaketh,” He said in verse 34. What the mouth speaks, in other words, comes straight from the heart.

Notice verse 35: “A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things.” Evil words and actions spring from an evil heart.

But Jesus also said good things will proceed out of our mouths if we take charge of our lives and start controlling what goes in! Righteousness in, righteousness out, you might say.

Paul taught this in Philippians 4:8: “Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things.”

People sometimes wonder why they can’t get rid of sinful thoughts. It’s generally because they aren’t diligently working to replace those thoughts with the things of God—whatsoever is true, honest, just, pure and lovely! The mind, like nature, abhors a vacuum. We have to fill our minds with what Paul wrote about in Philippians 4. Then there won’t be room for wrong thoughts.

In 2 Peter 3:1, Peter admonished God’s people to “stir up” their pure minds by studying his epistle. We’ve got to actively fill our minds with purity and holiness—and squeeze out the impurities of wickedness and vanity.

Peter continued, “That ye may be mindful of the words which were spoken before by the holy prophets, and of the commandment of us the apostles of the Lord and Saviour” (verse 2).

To meditate on God and His ways, we must regularly feed on, or study, His inspired Word (read all of John 6). Unless we are filling our minds up on God’s Word every day, how can we possibly be “mindful of the words” the prophets wrote—or the words of Jesus Christ and the apostles?

David wrote that he meditated on God’s Word “day and night” (Psalm 1:2). He didn’t confine his meditation to a simple 20-minute block of contemplative silence—or to a weekly stroll through the park. For David, meditating on God’s law was a constant process of setting his mind on the things of God (note the words “seek” and “set” in Colossians 3:1-2).

He obeyed the instruction God gave in Joshua 1:8: “This book of the law shall not depart out of thy mouth; but thou shalt meditate therein day and night.” In Psalm 119:97, David exclaimed, “O how love I thy law! it is my meditation all the day.”

In the Bible, the word meditate means to think about—to consider. Whatever fills our minds, in other words, is the subject of our meditation. David’s meditation, as he said in the psalm, was God’s law.

Concentrating on God’s law didn’t just come naturally for King David. He had to really work at it. In fact, because it’s so easy to meditate on the wrong things, David asked God for the strength and discipline needed to fill his mind with the right things. “Let the words of my mouth, and the meditation of my heart, be acceptable in thy sight, O Lord, my strength, and my redeemer,” David prayed in Psalm 19:14.

“Search me, O God,” he asked, “and know my heart: try me, and know my thoughts: And see if there be any wicked way in me, and lead me in the way everlasting” (Psalm 139:23-24).

In the spiritual war we wage against Satan and his demon army, the front line of our battlefield is in the mind. This is where we are engaged in a life-and-death struggle to resist against the devil’s powerful broadcast—the spirit of disobedience (Ephesians 2:1-2).

2 Corinthians 10:5 adds, “Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ.” Are we obedient to God in “all things” (2 Corinthians 2:9)—even in our thoughts? Or are we often taken captive by our own wicked and perverse reasoning?

A true Christian’s life-and-death struggle against sin starts in the mind (James 1:14-15). We have to constantly guard against carnal reasonings and lustful impulses—not allowing those invading thoughts and temptations to gain a foothold in the mind.

Attack those negative thoughts with the corresponding right thoughts, immediately and forcefully! Overcome the evil by replacing it with the good, as it says in Romans 12:21.

Put righteousness in. Then what comes out will be productive and useful!