The parable of the leaven is referring to the sin that works against the Kingdom.  Notice the pervasive nature of sin, how it spreads and infects everything.  What comes to mind is, "Whatever is not of faith is sin."  Whatever we think, say, or do that is not according to the leading of God is sin.  Some have claimed that God has led them into ways that conflict with Scripture.  This would not be possible since it would put the Holy Spirit in conflict with Himself.  Such claims are examples of just how easily people can fool themselves and how much we need the structure of the Church as revealed through Scripture.  There are safeguards built into God's pattern to prevent error and pride from springing up. 

Leaven can be simply not responding to the leading of God's spirit.  Notice that Jesus had just warned against the leaven of the Pharisees in the previous chapter.  This is carried further with the teaching of the straight (restricted) gate.  It is very restricted.  We are to follow Christ moment by moment, hearing His Voice and responding in obedience by the power of His grace.  Anything else is sin and will not yield fruit.  Immediately, He speaks of those who have been in His presence and heard His teaching but have worked iniquity.  The indication is that this is serious enough to thrust someone out of the Kingdom.