Have you ever heard someone say, "You will sin every day in thought or deed!"? How true is that saying? Under what circumstances might it be true? And under what circumstances might it be false? And is it possible to never sin? First, we need a definition for sin. Leviticus 4 is a good place to start. Notice that it says "whoever sins and breaks a commandment" multiple times. This means that the definition of sin is breaking God's commandments (that is, his law). And 1 John 3:4 also confirms that this is the definition of sin, as does the Apostle Paul, on multiple occasions. For instance, in Romans 7:7.
Now that we have a definition for sin that comes directly from the Bible, we can begin to answer the question of whether it's true that a Christian will sin every day in thought or deed. To start, let's reread something that Jesus said at the beginning of his ministry:
13 "You are the salt of the earth; but if the salt loses its flavor, how shall it be seasoned? It is then good for nothing but to be thrown out and trampled underfoot by men.
14 "You are the light of the world. A city that is set on a hill cannot be hidden. 15 Nor do they light a lamp and put it under a basket, but on a lampstand, and it gives light to all who are in the house. 16 Let your light so shine before men, that they may see your good works and glorify your Father in heaven. [Matthew 5:13-16; NKJV]
How much of a light are we if we sin every day? Aren't we just like the world if we do? And before anyone pulls the "but we confess that we sin" card, doesn't even the world admit that they do what is contrary to the ways of God? Some even take pride in it, and even sit in judgement of God's Law, rather than submitting to it. So if it is true that a Christian sins every day in thought or deed, then that Christian shows the world that there's no difference between believing in Jesus and not believing.
So if a Christian does sin every day in thought or deed, that Christian either (a) has not learned what is or is not sin, (b) has not learned to be careful so that he or she does not sin, or (c) does not care what God's commandments are and is therefore not actually a Christian. Jesus told us to be perfect (Matthew 5:48), so it is possible to never sin. But who has done so, except Jesus himself? Didn't Jesus also say to "sin no more"? Again, it is entirely possible for someone to never sin again!
What would it take for someone to never sin? First, it would take a perfect knowledge of what God's Law says about everything, followed by a perfect understanding of all that it says. It would also take a perfect understanding of which commands are currently impossible to keep (there are some commands which cannot currently be kept as prescribed). And if two or more commands come into conflict, which command take priority. Finally, that person would have to always be careful to never break a single command, ever! But who has ever done that except Jesus himself? That is exactly why we need to trust him so save us as he promised.
So, Jesus told us to stop sinning and to be perfect. But he also said that anyone who sins is a slave to sin (John 8:34). And that those who love him will keep his commandments (John 14:21) So the one who believes in Jesus will make every effort to not sin. What does this mean? It means the one who believes in Jesus will study God's Law, so that he or she knows what is right and what is wrong. That person will also try to implement what he or she has learned, so as to not do anything that is contrary to God's commandments. Finally, such a person will also be very careful not to break any of those commandments. The person that does all of this will rarely sin. This is the duty of everyone professing faith in Jesus.
Generally speaking, the teaching that someone will sin every day in thought or deed is accompanied by a teaching that, when examined closely, boils down to "you're going to sin anyways, so don't bother trying not to sin". This of course is contrary to everything that Jesus taught. The same people who teach this also try to correct Christians for their sin. Does anyone see the hypocrisy in that? If you teach "you can't not sin, so don't even try", then you shouldn't be surprised when someone follows that teaching. It does not matter if it is a big sin or a little sin. Neither does it matter if a person merely doesn't care if they sin, or if they actively seek to deliberately commit sin. If you teach "don't even try", and someone takes that to heart, you reap what you sow!
Now, what about if we do sin by accident or without knowing that we have sinned? That's where the grace of God comes in. For the one who believes in Jesus, the blood of Jesus covers these types of sins. When we become aware of our sins, we confess those sins and repent. And God forgives us of those sins.