Immanuel Kant (1724 - 1804) asked what it is that makes people behave morally, apart from achieving some happiness for themselves by doing so.
Kant thought that there are certain objective laws which people are 'duty bound' to follow when faced with a moral decision. He called them categorical imperatives. Kant said that using reason could discover them. We can only fulfil these laws if we have the freedom to do so and if God is there to help us.
Kant believed that this wasn't 'proof' of God's existence. He thought that proof lay with faith.
Strengths of the argument
The arguments support some things that theists already believe. The Bible teaches that God is good, expects certain standards of behaviour, will judge people and will also forgive them when they admit to doing wrong. A believer will understandably want to trace the origin of morality back to God.
Weaknesses of the argument
There are many alternative explanations for morality that have nothing to do with God, eg conscience might be entirely a product of the brain. If conscience is the voice of God, then why are there differences of opinion about issues such as abortion and euthanasia?
Not everyone believes in God, yet atheists and agnostics seem to understand the difference between right and wrong and to live good lives.
The idea of objective laws suggests that there is no room to consider the consequences of an action before deciding to take it.
The moral argument might suggest the existence of some sort of lawgiver, but it cannot prove the existence of God as traditionally understood.