To do this you will need a little bit of money but the profit can be huge. You will need a Prayer skill of 31 to do this.
So buy as many Unblessed Symbol as you want. Then head over to the Edgeville Monastery and go inside. Talk to Abbot Langley and ask him to join the Monastery. He will let you join. Then go up the ladder and talk to Brother Jared and have him bless your Unblessed symbols and they will turn into Holy Symbol. So now head back to the Edgeville bank and bank your Holy Symbols. Rinse and repeat as much as you want.
An Unblessed Symbol will go for about 50 – 60 gp and a Holy Symbol will sell for about 115, which is double what you paid for them so it is a pretty good profit. This method is also very fast to you can make a lot in an hour.
If you want to learn how to make even more gold, check out :