People stumble when they unexpectedly trip over something ahead of them.  Stumbling is dangerous and if the cause is the carelessness, ignorance or malice of other people, they are accountable.  Jesus used the ‘stumbling’ metaphor to warn the disciples that God would judge anybody who does anything to prevent people trusting in Him.  Jesus had been talking about entering the kingdom of Heaven, and had used a little child as an example of simple trust and confident dependence (Matthew 18:1-5).  Then He gave four serious warnings about putting obstacles in other people's way so that they would stumble and not enter God's kingdom.
 
The first warning was to anyone (religious leaders included) who would prevent children and adults with child-like faith from coming to Christ's kingdom and continuing to grow in faith (Luke 11:52).  The second warning was to worldly people - those who have not believed, and who also prevent others from moving forward into a holy relationship with God.
 
Jesus' other metaphor of the large millstone is equally obvious.  He was saying that the problem was so grave that the most radical action would need to be taken (Matthew 18:7).   Although their society disregarded children, and religious people disregarded people who followed Jesus, the Lord said that their attitude offended God greatly.  His punishment would be far more severe.
 
In God's mercy, not all stumbling leads to falling (Isaiah 40:30).  But Jesus wanted the disciples to understand that anything which obstructs the pathway of faith is such a serious matter that it needs radical action.  It is still true today.  The relationship between God and one of His 'little ones' (all of His children) is so precious that He will deal severely with those who seek to disrupt it.  The warning is designed to ensure that the gospel is not restricted to the religious, wealthy or influential but is released without obstruction to the 'little ones' – all those with no power or significance in this world, but who are precious to God.  May the way we live, and what we teach (James 3:1), be a positive encouragement to all who want to trust in Jesus (1 Timothy 4:16).