FAITH has tremendous power. For example, although Satan wants to kill us spiritually, faith enables us “to extinguish all the wicked one’s burning arrows.” (Eph. 6:16) With faith, we can face mountainlike problems. Jesus told his disciples: “If you have faith the size of a mustard grain, you will say to this mountain, ‘Move from here to there,’ and it will move.” (Matt. 17:20) Since faith has the potential for strengthening us spiritually, we do well to consider these questions: What is faith? How does our heart condition affect our faith? How can we strengthen our faith? And in whom should we put faith?​—Rom. 4:3.

Faith goes beyond simply believing or acknowledging the truth, for even “the demons believe [that God exists] and shudder.” (Jas. 2:19) What, then, is faith?

The Bible defines faith as having two aspects. First, “faith is the assured expectation of what is hoped for.” (Heb. 11:1a) If you have faith, you firmly believe that everything Jehovah says is true and will be fulfilled. For example, Jehovah told the Israelites: “If you could break my covenant regarding the day and my covenant regarding the night, to prevent day and night from coming at their proper time, only then could my covenant with my servant David be broken.” (Jer. 33:20, 21) Are you ever afraid that the sun might stop rising and setting in the sky, thus causing day and night to cease? If you do not doubt the physical laws that keep the earth rotating on its axis and orbiting the sun, should you doubt that the Creator of these laws can fulfill his word? Of course not!​—Isa. 55:10, 11; Matt. 5:18.

Second, faith is “the evident demonstration of realities that are not seen.” Faith is said to be “the evident demonstration,” or “convincing evidence,” of things that are invisible to the eye and yet real. (Heb. 11:1b; ftn.) In what way? Suppose a child asks you, ‘How do you know that air exists?’ Although you have never seen air, you would likely help the child to reason on the evidence that exists​—breathing, the effects of wind, and so on. Once the child is convinced of the evidence, he accepts the existence of what is invisible to him. Similarly, faith is based on solid evidence.​—Rom. 1:20.

Since faith is based on evidence, to have faith a person must first acquire “an accurate knowledge of truth.” (1 Tim. 2:4) But that is not enough. The apostle Paul wrote: “With the heart one exercises faith.” (Rom. 10:10) A person must not only believe the truth but also value it. Only then will he be motivated to exercise faith, that is, to act in harmony with the truth. (Jas. 2:20) A person who does not have heartfelt gratitude for the truth may reject even convincing evidence if he stubbornly holds to preconceived ideas or seeks to excuse his fleshly desires. (2 Pet. 3:3, 4; Jude 18) This is why in Bible times, not all who witnessed miracles developed faith. (Num. 14:11; John 12:37) God’s holy spirit produces faith only in people whose heart favors the truth over lies.​—Gal. 5:22; 2 Thess. 2:10, 11.

Among those who had outstanding faith was King David. (Heb. 11:32, 33) However, not everyone in David’s family had such faith. For example, on one occasion Eliab, David’s oldest brother, showed a lack of faith when he rebuked David for his concern about Goliath’s challenge. (1 Sam. 17:26-28) No one is born with faith; nor does anyone inherit faith from his parents, so David’s faith was a result of his own relationship with God.

In Psalm 27, David reveals how he came to have such strong faith. (Vs. 1) David meditated on his past experiences and on how Jehovah had dealt with his adversaries. (Vss. 2, 3) He deeply appreciated Jehovah’s arrangement for worship. (Vs. 4) David worshipped God along with fellow believers at the tabernacle. (Vs. 6) He earnestly sought out Jehovah in prayer. (Vss. 7, 8) David also wanted to be instructed in God’s way. (Vs. 11) So important was this quality to David that he rhetorically asked: “Where would I be if I did not have faith?”​—Vs. 13.

You can have faith like that of David if you imitate the mental attitude and habits reflected in Psalm 27. Since faith is based on accurate knowledge, the more you study God’s Word and Bible-based publications, the easier it will be to produce this aspect of the fruitage of God’s spirit. (Ps. 1:2, 3) Take time to meditate when you study. Meditation is the soil in which appreciation grows. As your appreciation for Jehovah deepens, so will your desire to exercise faith by worshipping him at congregation meetings and declaring your hope to others. (Heb. 10:23-25) Also, we demonstrate faith when we continue “to pray and not to give up.” (Luke 18:1-8) Therefore, “pray constantly” to Jehovah, trusting that “he cares for you.” (1 Thess. 5:17; 1 Pet. 5:7) Faith moves us to action, and our actions, in turn, strengthen our faith.​—Jas. 2:22.

On the evening before his death, Jesus told his disciples: “Exercise faith in God; exercise faith also in me.” (John 14:1) So we need to exercise faith not only in Jehovah but also in Jesus. How can you exercise faith in Jesus? Let us consider three ways.

First, view the ransom as God’s personal gift to you. The apostle Paul said: “I live by faith in the Son of God, who loved me and handed himself over for me.” (Gal. 2:20) When you exercise faith in Jesus, you firmly believe that the ransom applies to you, is the basis for forgiving your sins, offers you the hope of everlasting life, and is the greatest confirmation of God’s love for you. (Rom. 8:32, 38, 39; Eph. 1:7) This will strengthen you to keep negative feelings about yourself at bay.​—2 Thess. 2:16, 17.

Second, draw close to Jehovah in prayer on the basis of Jesus’ sacrifice. Because of the ransom, we can pray to Jehovah “with freeness of speech, so that we may receive mercy and find undeserved kindness to help us at the right time.” (Heb. 4:15, 16; 10:19-22) Prayer strengthens our resolve to resist the temptation to sin.​—Luke 22:40.

Third, obey Jesus. The apostle John wrote: “The one who exercises faith in the Son has everlasting life; the one who disobeys the Son will not see life, but the wrath of God remains upon him.” (John 3:36) Note that John contrasted exercising faith with being disobedient. Therefore, you exercise faith in Jesus when you obey him. You obey Jesus by following “the law of the Christ,” that is, all that he taught and commanded. (Gal. 6:2) You also obey Jesus by heeding the guidance he provides through “the faithful and discreet slave.” (Matt. 24:45) By obeying Jesus, you will have the strength to endure stormlike adversities.​—Luke 6:47, 48.

Once, a man cried out to Jesus: “I have faith! Help me out where I need faith!” (Mark 9:24) He had a measure of faith, but this man modestly recognized that he needed more faith. Like this man, all of us at some point will need more faith. And all of us can strengthen our faith now. As we have seen, we strengthen our faith when we study God’s Word and meditate on it, which will deepen our appreciation for Jehovah. Our faith will also grow stronger when we​—along with fellow believers—​worship Jehovah, publicly declare our hope, and persevere in prayer. Moreover, when we strengthen our faith, we receive the greatest reward of all. God’s Word urges us: “Beloved ones, build yourselves up on your most holy faith . . . in order to keep yourselves in God’s love.”​—Jude 20, 21.