God tells us to pursue holiness (He 12:14a), and that we’ll never see God without it. (14ba) This doesn’t contradict the Gospel, it reveals the Gospel.
To be holy is to be dedicated, entirely set apart for something or someone. God sets apart a chosen people for Himself, deciding we should be holy (Ep 1:4), and we engage with Him and participate in how this plays out. (Php 2:12-13)
God calls us to be holy because He is holy. (1Pe 1:16) He both calls us to be like Him, and He intends to transform us into His likeness. (Ro 8:29)
Holiness is not merely an attribute of God, distinct from His other attributes, it informs and imbues all of His attributes: His love is holy love, His justice is holy justice, His mercy is holy mercy, etc. This is how holiness also works in us: it permeates and colors our every thought and action. (1Pe 1:15)
In being set apart to God, we are being separated from sin, for sin is explicitly contrary to the nature of God (1Jn 1:5): He cannot violate the standard of righteousness because He is that standard by His very nature. Whenever and however we sin, we are departing from the nature of God, from holiness.
So, cleansing ourselves from all filthiness of the flesh and spirit is how we pursue and perfect holiness in the fear of God (2Co 7:1), daily renewing our minds in His truth. (Ro 12:2) This is the journey of sanctification (Jn 17:17), and all of God’s children travel this way. (1Th 5:23-24)
Just as we strive to enter the kingdom of God (Lk 13:24), we strive against sin (He 12:4), diligently adding virtue to our faith (2Pe 1:5), to be filled with all the fullness of God. (Ep 3:19)
Holiness is not merely separation from sin; it is the complete dedication of ourselves to God, which necessarily requires separation from everything contrary to His nature. Holiness means I am a living sacrifice (Ro 12:1); I am available for God to do with me as He pleases; I no longer belong to myself. (1Co 6:19-20)
Knowing God has a unique purpose for my life that is according to His design and pleasure frees me from the trap of comparing myself with others (Jn 21:21-22) to focus on running the race He has designed me for and called me to. (1Co 9:24) He is all the reward I need. (Ps 73:25-26)