One of Satan's most deceptive tools is causing us to believe that we cannot really know the truth. He tells us that we cannot understand the difference between right and wrong, truth and error; and that justifies sinning. God gave us the capacity to know the truth and designed His Word to match our capacity to understand.
Satan replaces God's exclamation points with question marks. God warned Adam and Eve not to eat of a certain tree, and He included an emphatic penalty: "For in the day that thou eatest thereof thou shalt surely die." (Genesis 2:17) When Satan seduced Eve to eat, he began by planting a subtle doubt in her mind. He asked, "Yea, hath God said, 'Ye shall not eat of every tree of the garden?'" Then, he denied the certainty of the penalty and presented sin as a great advantage withheld by God's Word (3:4-5).
We can know about God. God filled the universe with the evidence of His reality. "For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even His eternal power and Godhead; so that they are without excuse." (Romans 1:2) Though God is invisible, by reflecting on the creation, we can clearly see and understand His existence. All life's blessings bear clear testimony to God's love and care for us (Acts 14:17). Such evidence makes the doubt and denial of atheists and agnostics utterly without excuse.
We can understand God's offer of salvation in Christ. "For the wages of sin is death; but the gift of God is eternal life through Jesus Christ our Lord." (Romans 6:23) Jesus teaching and His example made the truth accessible. He is so much the perfect embodiment and revelation of all truth that he is called "the Word" (John 1:1,18). His miracles confirmed His claim to be God's Son, but some men hardened their hearts, closed their eyes, and doubted (John 3:2,16; 10:24-25). As Jesus taught men about salvation from sin, He said, "If ye continue in My word, then are ye My disciples indeed; and ye shall know the truth, and the truth shall make you free." (John8:31-32) Yes, we can know the truth!
We can know about baptism. We are saved from our sins when, as Christ authorized, we believe in Him, repent of sin, confess His name, and are immersed in water. Jesus said that "all the world," "every creature," and "all nations" can understand the necessity of faith, repentance, and baptism as conditions of pardon (Mark 16:16; Luke. 24:47). We can understand that immersion involves a burial in water, not merely sprinkling or pouring water (Colossians 2:12). Sinners come to Christ in order to be "baptized into one body," the body (or church) of Christ (1 Corinthians 12:13). The "one baptism" puts us into the "one body"--not some human denomination, but the church whose name, doctrine, and practice are found in the Bible (Ephesians 4:4-6).
We can know about singing, versus instrumental music. God has always given His people a clear and understandable pattern for acceptable worship. His word is an unalterable pattern of truth (Exodus 25:9; 2 Timothy 1:13). The Bible clearly authorizes singing in New-Testament worship (Ephesians 5:19; Colossians 3:16). Instrumental music is added by human doctrine, will worship, and departure from the faith (Matthew 15:8-9; Colossians 2:23; 1 Timothy 4:1).
Sin and error thrive on doubting God's revealed truth. Some men are "ever learning, and never able to come to the knowledge of the truth," because they look for ways around the truth, rather than looking for the truth (2 Timothy 3:7). Doubt and denial are spread by smug questions such as, "Do you claim to know all there is to know?" or, "Do you think you can never make a mistake?" We do not have to know everything or be above making mistakes to know that God exists, salvation is only in Christ, baptism is a condition of pardon, and the New-Testament church sang but did not use instrumental music. We know these things by reading God's word and refusing to replace His exclamation points with Satan's question marks.