I went to Atlas' temple first.
In order to get there go to the right and jump on the stone pads you pulled out earlier.
Jump to the right and with some help from the ledges you should get to some slopes. Jump on the first one, slide down just enough to reach the ledge in front and jump.
Then just move to the front side and use Shift to drop down. Lara will grab the ledge below and you can then reach the entrance of the temple.
The entrance itself is quite tricky (the same it's used for all temples). It has a quite simple mechanism. Only a door and a set of spikes open so you'll need to use the hole on top of the door to enter.
First, flip the switch and jump in front of the door. Lara will grab on the ledge. When the door reaches up, you will hear 3 sounds. Right after the third, the door will close abruptly. This is why, after you hear the third sound, just jump. Lara will grab on the ledge and you can go in. If you don't get this from the first time, just try again.
It seems Atlas' temple is protected by a huge pit. Turn the wheel in front to pull out a platform to walk upon and then hurry up, as it retracts in time.
Notice there are two shooting targets on each side of Atlas' statue, you will need to shoot them in order to retract the spikes which protect the key you're looking for. However, first go on the left side.
You'll notice a series of ledges which lead to a switch. Flip it and a pole will exit from the wall which you can use to jump over the pit.
Notice on the other side there is another switch with a medikit and an artifact. You can't reach them yet though.
Now go to a safe distance from the statue and shoot the two targets. Right after that the giant globe will fall. You will need to be VERY fast and run, jump and swing over the pit. You might need to do this a couple of times, but if you pay a bit of attention it shouldn't be a problem.
Now get back to the side with the statue by using the wheel like the first time and pick up the now accesible key.
You just found the first key: The key of Atlas!
In order to get back, climb on the left side using the broken column, pick up the medikit and eventually the artifact then flip the switch. It will have the same effect as the wheel, so hurry up.
At the exit, the same problem with the door, but now you won't hear the sounds. So in order to get in, hold on the door while it goes up, jump to the thin pole to your back and when the door is down jump back to grab the hole ledge.
As you get the key from each temple, a block of stone will come out in from of it.