Grounds and Conditions of Atonement

"Grounds", from Dictionary.com, means "the foundation or basis on which a belief or action rests; reason or cause". And a "condition" is a "restricting, limiting, or modifying circumstance; a circumstance indispensable to some result; prerequisite; that on which something else is contingent; something demanded as an essential part of an agreement; provision; stipulation". A condition may be taken to mean a sine qua non, a "not without which". It is helpful to bear these definitions in mind to avoid confusing the "grounds" and "conditions" of a particular action.

Given these definitions, what are the grounds and conditions of the Atonement?

"For God so loved the world that He gave his only begotten Son, that whoever believes in Him should not perish, but have everlasting life."

The ground, or ultimate purpose or reason, for the Atonement, is God's love. A condition, or sine qua non, of entering into the benefits of the Atonement (i.e., not perishing, but having everlasting life) is believing in Jesus.