Prophet Muhammad informed us that when a person recites the shahada for the first time, firmly believing in it, all of their past sins are wiped away and all the good that they have done remains.
In other words, upon declaring God to be the only one worthy of your devotion and accepting the revelation God sent through His messenger Muhammad, you become as pure and sinless as a newborn child.
Prophet Muhammad also informed us that there are different gates of paradise in the life to come and each believer earns entry through one or more of the gates by the merit of their actions. There is the gate for the people who excelled in giving charity, another for the people who engaged prayers and worship, gate for the people who stood for justice and defended the truth, gate for those who fasted and warded off temptations etc.
The Prophet told us if anyone of you would like to gain entry through all the gates of paradise – in other words reach the highest level of virtue and gain the pleasure of Your Creator – then recite the shahada every time you finish performing the ritual purification and all the gates of paradise will be open for you.
Many scholars have said that the person who recites the shahada regularly will be able to say it with ease at the moment of death. And the Prophet told us that anyone who at the moment of death utters the words of the shahada will definitely enter paradise.
It was the practice of the Prophet that when he would say the shahada he would raise his index finger on his right hand. This became the symbol of declaration of Oneness of God. Many Muslims when they are reminded of the Glory and Grandeur of the Creator, automatically raise the index finger as a physical gesture of declaring the shahada.