"It is a faithful saying: For if we be dead with him, we shall also live with him: If we suffer, we shall also reign with him: if we deny him, he also will deny us: If we believe not, yet he abideth faithful: he cannot deny himself." (2 Timothy 2:11-13)
This saying may have been a song or other memory device that Paul recommended as a summary of doctrine. It expresses important elements of saving faith. First, Christ's vicarious death gives us eternal life in Him. We "who were dead in trespasses and sins" (Ephesians 2:1) have been created "in righteousness and true holiness" (Ephesians 4:24) and have "passed from death unto life" (John 5:24).
Second, standing with Christ in this life attests to our reigning with Him in the next. The "persecutions and tribulations that |we| endure" are a "manifest token of the righteous judgment of God, that |we| may be counted worthy of the kingdom of God, for which |we| also suffer" (2 Thessalonians 1:4-5).
Also, denying Christ in this life will insure that He will deny us for eternity. "Whosoever shall deny me before men, him will I also deny before my Father which is in heaven" (Matthew 10:33). "He that overcometh, the same shall be clothed in white raiment; and I will not blot out his name out of the book of life, but I will confess his name before my Father, and before his angels" (Revelation 3:5).
Finally, even our unbelief will not affect Christ's faithfulness. "For all the promises of God in him are yea, and in him Amen" (2 Corinthians 1:20). "Thy word is true from the beginning: and every one of thy righteous judgments endureth for ever" (Psalm 119:160). "For I am the LORD, I change not" (Malachi 3:6).
May this faithful saying be your foundation in faith. It is a guide to salvation and an anchor for eternity. HMM III