Trust is necessary when you wish to control your horse with ease. Certainly, you can still ride a horse that you are uncomfortable with but it will be harder to control a horse when it feels threatened or uncomfortable around you.
This is especially true if the horse you chose has had a bad past with other owners. This can make it difficult to build the bond between the both of you. But with perseverance, you can learn to become close.
If your horse has never been mistreated but is skittish by virtue, it will also make it harder for the horse to have faith in you. All it takes is some time together. You will reconsider how you approach him. Make sure you move slowly and approach the horse from the left. If you are walking towards him with anything in your hands, stretch it out in front of you to show him there is nothing to be afraid of whatsoever.
Do not wear baggy outfits when you are moving close to a skittish horse. The wind could pick up your shirt or pants and cause a sudden clothing flapping noise that will spook him. Once they associate you with any kind of fear, it will take much longer for the horse to trust you at all. The horse should never shy away from its owner over anything.
No one should push a horse to do something before he is totally confident. You will want your horse to trust and depend upon you completely before you ask him to jump or go down an unfamiliar terrain. Let him gain assurance in himself by trying easier obstacles and running down wide open courses. If he trips and regains his footing, it might make it that much harder for him to have faith in you while attempting it again and you will also doubt his abilities while attempting it again. So long as you know that each of you can trust the other, you will have a safe and successful ride.
If you have a horse that has been abused, it can be nearly impossible to gain the horses trust. You shouldn't give up hope, however. Plenty of horses have come back around after hours and hours of patience and love.
A good way to start with a horse that has been abused is to spend hours talking and sitting with him. Speak in a mellow and steady tone and carry treats with you. Do not attempt to ride the horse or force him to do things he is unwilling to try. It could take days before he is afraid at the very sight of you. Once he doesn't seem to mind your company, you can try to approach him. A frightened horse is a very dangerous animal, so be sure that you do not corner him. Offer him a treat and try to stroke his muzzle. If he eats the treat from your hands, you are making great strides!
If you can touch your horse and he lets you pat him, you are nearly there. Do not attempt to get on a horse that has been mistreated previously without the supervision of professional trainers who are skilled with dealing with such horses. You do not want to get on the back of a horse who will trust you on the ground, but utterly lose faith once you are on his back.