previa invocación de Dios Todopoderoso

18:39 Mar 22, 2015 

People were still putting this sort of thing in Spanish wills in the first half of the twentieth century.

For a long time the boilerplate opening to a will in English was:

"First and principally, I commit my Soul into the hands of Almighty God, and my body to the earth..."

With variants: "commend ... to the mercy" for "commit ... into the hands", etc. But this is what people practically always said. Perhaps it is the way to go here. At any rate, if you want to do it more literally "invocación", theologically speaking, really means calling on God to assist you (somewhat along the lines of Robert's suggestion).

17:18 Mar 22, 2015 

I wouldn't bet on it being a modern will e.g.

"y teniendo, á mí juicio, capacidad legal para testar, como así parece también á los testigos presenciales, previa invocación de Dios Todopoderoso y protesta de profesar la religión Católica, Apostólica Romana, otorga su testamento en esta forma.

Primero: Quiere que su cadáver sea vestido con hábito de San Francisco y enterrado en el suelo del cementerio de esta población ..."

OR
" en el libre uso de sus facultades intelectuales y habla expedita para poder testar, como así aseguramos los testigos y yo, el Notario, los cuales conocemos a la señora testadora, previa invocación de Dios Todopoderoso y protesta de profesar la Religión Católica, Romana, ordena su testamento en la forma siguiente:

17:10 Mar 22, 2015 

I've never seen this in a modern will in Spain, but I suppose one is certainly free to include this or a similar expression in his "últimas voluntades" (and Melanie obviously has found one that does).

17:06 Mar 22, 2015 

It seems very dated in English e.g. "In the name of God, Amen. I, XXX, being of sound mind and body, ... make and set out this my testament in the following manner. First, I leave and commend my soul to Almighty God, my creator and my saviour, to the Blessed Virgin Mary his mother, and to all the saints, and my body to be buried in ...