Keep it in context since this is about all the steps the new believers had taken in being His disciples in the beginning for which the exhortation to continue to perfection is given. In other words, not laying again that foundation that new believers had gone through.
Hebrews 6:1Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God, 2 Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment. 3 And this will we do, if God permit.
So the point of Hebrews 6:4-6 is to testify that there is no need for that "laying again the foundation" and why, because we can't crucify Jesus Christ again. When we read on, we see the reality for those that do not go on to perfection, but to sowing to the works of the flesh.
Hebrews 6:7 For the earth which drinketh in the rain that cometh oft upon it, and bringeth forth herbs meet for them by whom it is dressed, receiveth blessing from God:8 But that which beareth thorns and briers is rejected, and is nigh unto cursing; whose end is to be burned.9 But, beloved, we are persuaded better things of you, and things that accompany salvation, though we thus speak.
So if you apply the metaphor to 1 Corinthians 3:10-17 where the believer is judged by what they had built on that foundation, and yet the spirit is saved when their bodies are destroyed by physical death, you can see Hebrews 6:7-8 as inferring that life has been received as the ground had received the rain, but what grew there was to be subjected to be burned, but the rain is still in the ground as life received for why there is no need to lay again the foundation that the believer went through at the beginning, but to call to repentance in order to continue on to perfection.
If a believer errs from the truth and have their faith overthrown, and become a former believer, they are still called to depart from iniquity because that foundation cannot be removed and neither can His seal. By discerning with Him at that throne of grace the lies that overthrew their faith, He will help them depart from iniquity and all other iniquities to be that vessel unto honor in His House when the Bridegroom comes.
2 Timothy 2:18 Who concerning the truth have erred, saying that the resurrection is past already; and overthrow the faith of some. 19 Nevertheless the foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, let every one that nameth the name of Christ depart from iniquity. 20 But in a great house there are not only vessels of gold and of silver, but also of wood and of earth; and some to honour, and some to dishonour. 21 If a man therefore purge himself from these, he shall be a vessel unto honour, sanctified, and meet for the master's use, and prepared unto every good work.
Although they deny Him, and carnal believers & religious believers can engage in works of iniquity that denies Him, they will be denied by Him when He comes as the Bridegroom and left behind to be resurrected after the great tribulation in becoming vessels unto dishonor in His House, because He still abides. That is why former believers are called to depart from iniquity so they too may be received by the Bridegroom as vessels unto honor in His House which is the eternal glory that comes with our salvation in Jesus Christ by looking to the author & finisher of our faith to help them lay aside every weight and sin in running that race for the high prize of our calling.
2 Timothy 2:10 Therefore I endure all things for the elect's sakes, that they may also obtain the salvation which is in Christ Jesus with eternal glory. 11 It is a faithful saying: For if we be dead with him, we shall also live with him: 12 If we suffer, we shall also reign with him: if we deny him, he also will deny us: 13 If we believe not, yet he abideth faithful: he cannot deny himself. 14 Of these things put them in remembrance, charging them before the Lord that they strive not about words to no profit, but to the subverting of the hearers. 15 Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth.
So as led by the Spirit, one can continue to call for former believers to repent by going to Jesus for help with their overthrown faith to hope for God to recover them from the snares of the devil by seeing the truth in His words as kept in the KJV.
24 And the servant of the Lord must not strive; but be gentle unto all men, apt to teach, patient, 25 In meekness instructing those that oppose themselves; if God peradventure will give them repentance to the acknowledging of the truth; 26 And that they may recover themselves out of the snare of the devil, who are taken captive by him at his will.
