According to Mormons, what is the Holy Spirit of Promise mentioned in Ephesians?

Question

Ephesians 1:13 (KJV) mentions being sealed by the Holy Spirit of Promise:

In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,

I can't find that term anywhere else in the Bible. So what is the Holy Spirit of Promise?

It does appear several times in the Doctrine and Covenants so I welcome answers with an LDS perspective. (For example, the D&C passages are about marriage; how does that relate to the context in Ephesians where one is sealed with the Holy Spirit of Promise after hearing the word of truth?)

Answer 1

The Holy Ghost is the Holy Spirit of Promise.

"Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Ghost, he hath shed forth this, which ye now see and hear." (Acts 2:33)

He confirms as acceptable to God the righteous acts, ordinances, and covenants of men. The Holy Spirit of Promise witnesses to the Father that the saving ordinances have been performed properly and that the covenants associated with them have been kept. 1

When Paul wrote that the Saints had been “sealed with that holy Spirit of promise”, he meant that they had been promised eternal life even though they were still living in mortality. When people are sealed by the Holy Spirit of Promise, the Holy Ghost ratifies them as celestial inheritors even though they are mortal. This doctrine is sometimes referred to as having one’s calling and election made sure or receiving the Second Comforter (see 2 Peter 1:4–19; D&C 132:6–7; Bruce R. McConkie, Doctrinal New Testament Commentary, 2:493–95; History of the Church, 3:379–80).2

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^{1 The Guide to the Scriptures: Holy Spirit of Promise
2 Chapter 44: Ephesians. New Testament Student Manual, 2014}