Because it’s the power of God unto salvation. It’s what Paul based all of his preaching on.
Just my own view, I believe once we understand our salvation. To continue to repeat that understanding would show a lack of faith.
2 Tim 1:14 Refer's to the Holy Spirit as a Good Deposit.
2 Cor 5:5, Eph 1:14 & 2 Cor 1:22. Refer to the Holy Spirit as "the earnest" = Deposit or Pledge.
Eph 1:13, 2 Cor 1:22 & Eph 4:30 Refer to the Holy Spirit as a Seal.
2 Tim 1:14 The Holy Spirit who dwells within us, guard the good deposit entrusted to you
(NOTE: The Holy Spirit dwelling within us. Is God's good deposit given us until the day of redemption)
2 Cor 5:5 God, who also hath given unto us the "earnest" of the Spirit
(NOTE: God's Holy Spirit dwelling within us is the earnest. Earnest translated from the Greek word: arrabon. Also means a deposit/pledge. In modern Greek arrabona is an "engagement ring.")
2 Cor 1:22 Who hath also "sealed us" & "given the earnest" of the Holy Spirit in our hearts
(NOTE: God has placed His Holy Spirit in the believers heart. As a DEPOSIT/PLEDGE & SEAL, until the soon coming day of our final redemption)
Eph 1:13 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,
(NOTE: You heard about Christ's finished sin redemptive payment & resurrection. When you believed/trusted, Christ sealed/baptized you with His Holy Spirit).
Eph1:14 Which is the "earnest" of our inheritance until the redemption of the purchased possession
(NOTE: The Holy Spirit by which only Christ baptizes/seals & indwells us. Is God's earnest = a down paymnt/pledge)
Eph 4:30 grieve not the holy Spirit of God, whereby ye are "sealed" unto the day of redemption
(NOTE: Believers are SEALED with God's Holy Spirit. Christ's pledge/promise that he will come for us)
1 John 5:13 13 These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
(NOTE: That you may know ""PRESENT TENSE"" that you have eternal life)
Romans 11:29 For the gifts and calling of God are without repentance.
(NOTE: God doesn't go back on His Word or Promises)