What is Sin?

According to Matthew 5:28, anyone who looks at a woman lustfully has already committed adultery with her in his heart. i.e., Just a wrong thought towards anyone in your heart is considered as Sin.

Sin is the only thing in this earth that separates us from God.

Matthew 5:8 tells Blessed are those who are pure in heart, for they shall see God. Example - Jesus is sinless through out his life. So he's able to see the father God. Sin is so powerful that at one point of time Jesus himself felt that he is separate from God. (Matthew 27:46)

In this world there is no one sinless(except JESUS). In order not to become prey of sin, we should follow two things in our life.

1. We should guard ourselves by Praying(Matthew 26:41).
2. We should obey everything what Jesus had commanded us(Matthew 28:20).

When we follow these two, God is always with us (Matthew 28:20 - Promise of his Presence).