Why the Gospel is a Faithful Saying

2 Timothy 2:8 Remember that Jesus Christ of the seed of David was raised from the dead according to my gospel: 9Wherein I suffer trouble, as an evil doer, even unto bonds; but the word of God is not bound. 10Therefore I endure all things for the elect's sakes, that they may also obtain the salvation which is in Christ Jesus with eternal glory. 11It is a faithful saying: For if we be dead with him, we shall also live with him: 12If we suffer, we shall also reign with him: if we deny him, he also will deny us: 13If we believe not, yet he abideth faithful: he cannot deny himself.