Colossians 1:23 If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister;
Faith is not a one-time thing that you do the day you are spiritually born. Faith is something that you must use on a continual basis if you desire to be spiritually strong. The more you continue in the faith, the more grounded you are and the harder it is for Satan to cause you to stumble.
A person does not go out and exercise only once in their lifetime. If they did, they probably would be overweight, in bad health, and they would not be in a position to react very quickly should the need arise. Just as a person must exercise on a regular basis in order to stay in physical shape, so too must they continue in the faith on a regular basis to stay in spiritual shape.