Some people believe that there is no karma, as they think that there is no witness to our actions and so how can we suffer or enjoy based on our past actions.
God is the witness to all our actions. He is present in the heart and thus the overseer of all our activities. On the Day of Judgment, which is immediately at death, the perfect witness and judge (God) will sentence you without making any mistakes.
Many times we will attempt to do certain things and sometimes it happens and sometimes it doesn’t. This is due to God. He is the permitter, and without the sanction of Lord Krishna (God), nothing can happen. We can make plans, but only Lord Krishna can make them come true.
“I am seated in the hearts of all living beings, from Me comes memory, knowledge, and forgetfulness. I am the compiler, knower of the Vedas, and known by all the Vedas.” (Lord Krishna, Bhagavad-Gita 15.15)
Even our memory, knowledge, and forgetfulness are due to the grace of God.
Srimad Bhagavatam, Canto 6, Ch1, TEXT 42
sūryo ‘gniḥ khaṁ marud devaḥ
somaḥ sandhyāhanī diśaḥ
kaṁ kuḥ svayaṁ dharma iti
hy ete daihyasya sākṣiṇaḥ
The sun, fire, sky, air, demigods, moon, evening, day, night, directions, water, land and Supersoul Himself all witness the activities of the living entity.
PURPORT – By HDG A.C Bhaktivedanata Swami Prabhupada
The members of some religious sects, especially Christians, do not believe in the reactions of karma. We once had a discussion with a learned Christian professor who argued that although people are generally punished after the witnesses of their misdeeds are examined, where are the witnesses responsible for one’s suffering the reactions of past karma? To such a person the answer by the Yamadūtas is given here. A conditioned soul thinks that he is working stealthily and that no one can see his sinful activities, but we can understand from the śāstras that there are many witnesses, including the sun, fire, sky, air, moon, demigods, evening, day, night, directions, water, land and the Supersoul Himself, who sits with the individual soul within his heart. Where is the dearth of witnesses? The witnesses and the Supreme Lord both exist, and therefore so many living entities are elevated to higher planetary systems or degraded to lower planetary systems, including the hellish planets. There are no discrepancies, for everything is arranged perfectly by the management of the Supreme God (svābhāvikī jñāna-bala-kriyā ca).