I think the Second Coming is the awakening of the awareness of our unity with God, and by extension with everyone and everything. This awareness is also known as "Christ", "Christhood", "Christ Consciousness" or "the kingdom of God, or heaven." The Buddhist equivalent would be the attainment of Buddhahood or Nirvana. In secular terms, it would be the attainment of unitive consciousness or awareness.
There were, are and will be certain highly evolved individuals who have attained this awareness to an extraordinarily high degree, and able to facilitate it in other people. Jesus the Christ and Gautama the Buddha are only two of the most famous examples. But it is not they who achieve the transformation, but the mind of the transformed. Every individual mind has to transform itself, with the help of its own, innate creative or transcendental powers. The aforementioned masters can only facilitate transformation. They cannot bring it about in other people.