Our constitution specifically provides for different forms of oath for different functionaries of our political system. This has been done in keeping with the different designations, their functions, and the nature of power they enjoy and the dignity of the post.

For the oath of office for a minister for the union he vows to bear true faith and alligance to the constitution. Here he acts as the constitutional functionary. When he takes the oath for secrecy he vows not to communicate directly or indirectly or reveal to any person.

A judge is required to be honest to his post thus he swears to perform the duty without fear or favour, affection or ill will.

A minister vows to discharge his function and his duties. Thus he vows by that.
Thus based on the nature of function and power, dignity and integrity of the post the different functionaries take their oath differently.