Some may say just us doing everyday magic(k) will bend the fabric of reality in a way. As far as summoning a God or Goddess I'm sure they will come and visit one of your spells whenever they feel the need, not just because you want them there.
Summon Loki? Impossible. Loki is chained in a hidden cave deep inside Jotunheim. Bound with three mill stones; one around his neck, one at his chest and one at his knees, and to ensure that he would not break free, the gods tied him wit the intestines of his son Nafre, and not until Ragnarok shall he be free again.