God hath spoken in His holiness; I will rejoice… – Psalm 108:7

It sometimes happens that our greatest difficulty in believing a promise of God lies in His holiness. There is, for instance, a promise of pardon to the soul that believeth in Jesus. We think of stern justice, with her majestic yet severe look. In our heart of hearts, we reverence her, and we ask, “How can God be just, and yet the Justifier of the ungodly…Can this holy God really mean to receive such sinners as we are whose very clothes, as Job says, do abhor us? Can He purpose to bring us to His own right hand in glory that we may be among the courtiers in His heavenly kingdom?” Yes, He does mean to do even that; yet the thought of His purity makes us wonder how it can be done.

(God) knew all that David was, and all that David would be; yet He saw it to be consistent with His infinite perfections to make, even with such a man, “an everlasting covenant, ordered in all things and sure;” and, beloved brethren, when the Lord entered into covenant with Christ concerning those whom He gave to Him to be His portion forever, and when, in that covenant, He wrote down blessings exceedingly great and precious, and made promises so vast that we cannot at present form any estimate of their full value, He knew quite well what He was doing, and He did it, knowing all about your doubts and fears concerning your sinfulness and His own holiness. And now, without in the least marring His perfect purity, and inflexible justice, “God hath spoken in His holiness” to poor lost sinners and said that He will save all of them who trust in Jesus Christ, His Son; and He has also “spoken in His holiness” to His poor imperfect children, and said that He will bless them, and that He will not turn away from them to do them good. This is the covenant that He hath made with His people: “A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you a heart of flesh. And I will put My Spirit within you, and cause you to walk in My statutes, and ye shall keep My judgments, and do them. And ye shall be My people, and I will be your God.” All this, which “God hath spoken in His holiness,” He will do without obscuring that wondrous attribute, or marring the glory of His adorable perfections. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/2864.cfm