In satyuga it was a devotee's test while in tretayug it was God's test..how? In satyuga when Hiranyakashyapu asks prahlaad is your god also in this stone pillar? Prahlaad says yes and he compels god to come out of that stone pillar because of his devotion.. In tretayug its God 's test that whether he can with his grace turn a stone into life form ? That is ahilya who has been cursed and turned into a stone ..can she be turned again into a human form? And yes the lord is so merciful ..he touches her with his merciful lotus feet and she comes back to life again