Albert Barnes on Hebrews 10:23

Let us hold fast the profession of our faith without wavering. To secure this was one of the leading designs of this epistle, and hence the apostle adverts to it so frequently. It is evident that those to whom he wrote were suffering persecution, Heb 12 and that there was great danger that they would apostatize. As these persecutions came probably from the Jews, and as the aim was to induce them to return to their former opinions, the object of the apostle is to show that there was in the Christian scheme every advantage of which the Jews could boast; everything pertaining to the dignity of the great Founder of the system, the character of the High Priest, and the nature and value of the sacrifices offered; and that all this was possessed far more abundantly in the permanent Christian system than in that which was typical in its character, and which were designed soon to vanish away. In view of all this, therefore, the apostle adds that they should hold fast the profession of their faith, without being shaken by their trials, or by the arguments of their enemies. We have the same inducement to hold fast the profession of our faith--for it is the same religion still; we have the same Saviour, and there is held out to us still the same prospect of heaven.

For he is faithful that promised. To induce them to hold fast their profession, the apostle adds this additional consideration. God, who had promised eternal life to them, was faithful to all that he had said. he arrangement here is,

(1.) That since God is so faithful to us, we ought to be faithful to him.

(2.) The fact that he is faithful is an encouragement to us. We are dependent on him for grace to hold fast our profession. If he were to prove unfaithful, we should have no strength to do it. But this he never does; and we may be assured that all that he has promised he will perform. To the service of such a God, therefore, we should adhere without wavering. Comp. 1Cor 10:13.

(a) "he is faithful" 1Thes 5:24