J.I. Packer on the Perseverance of the Saints

J. I. Packer, Knowing God, 275: 

God is adequate as our keeper. "Nothing...can separate us from the love of God," because the love of God holds us fast. Christians "are kept by the power of God through faith unto salvation" (1 Pet 1:5), and the power of God keeps them believing as well as keeping them safe through believing. Your faith will not fail while God sustains it; you are not strong enough to fall away while God is resolved to hold you.

One of the great mysteries of Christianity is that if you are a believer through faith in Christ, God is resolved to hold you.