To make the statement to that effect. And the seal is the Holy Spirit, who indwells every believer. That is God's promise or pledge FOR the day of redemption.
This is irrefutable. God have His Holy Spirit to all believers as a pledge for the day of redemption.
So now, the question is: Does God renege on His promises?
Where did God say that once a person receives the Holy Spirit there is nothing that they can do that will ever change that? Paul said,
KJV 1 Corinthians 10:1 Moreover, brethren, I would not that ye should be ignorant, how that all our fathers were under the cloud, and all passed through the sea;
2 And were all baptized unto Moses in the cloud and in the sea;
3 And did all eat the same spiritual meat;
4 And did all drink the same spiritual drink: for they drank of that spiritual Rock that followed them: and that Rock was Christ. {followed...: or, went with them}
5 But with many of them God was not well pleased: for they were overthrown in the wilderness.
6 Now these things were our examples, to the intent we should not lust after evil things, as they also lusted. {our...: Gr. our figures}
7 Neither be ye idolaters, as were some of them; as it is written, The people sat down to eat and drink, and rose up to play.
8 Neither let us commit fornication, as some of them committed, and fell in one day three and twenty thousand.
9 Neither let us tempt Christ, as some of them also tempted, and were destroyed of serpents.
10 Neither murmur ye, as some of them also murmured, and were destroyed of the destroyer.
11
Now all these things happened unto them for ensamples:
and they are written for our admonition, upon whom the ends of the world are come. {ensamples: or, types}
12 Wherefore let him that thinketh he standeth take heed lest he fall. (1Co 10:1-12 KJV)
Paul said that the things that happened to God's people in the OT were written for our example and admonition that we don't behave as they did, so that thinking he stands one may fall.
Given that, let's look at what God did say.
KJV Joshua 1:1 Now after the death of Moses the servant of the LORD it came to pass, that the LORD spake unto Joshua the son of Nun, Moses' minister, saying,
2 Moses my servant is dead; now therefore arise, go over this Jordan, thou, and all this people, unto the land which I do give to them, even to the children of Israel.
3 Every place that the sole of your foot shall tread upon, that have I given unto you, as I said unto Moses.
4 From the wilderness and this Lebanon even unto the great river, the river Euphrates, all the land of the Hittites, and unto the great sea toward the going down of the sun, shall be your coast.
5
There shall not any man be able to stand before thee all the days of thy life: as I was with Moses, so I will be with thee: I will not fail thee, nor forsake thee.
6 Be strong and of a good courage: for unto this people shalt thou divide for an inheritance the land, which I sware unto their fathers to give them. {unto this...: or, thou shalt cause this people to inherit the land, etc}
7
Only be thou strong and very courageous,
that thou mayest observe to do according to all the law, which Moses my servant commanded thee:
turn not from it to the right hand or to the left, that thou mayest prosper whithersoever thou goest. {prosper: or, do wisely}
8
This book of the law shall not depart out of thy mouth; but thou shalt meditate therein day and night, that thou mayest observe to do according to all that is written therein: for then thou shalt make thy way prosperous, and then thou shalt have good success (Jos 1:1-8 KJV)
God told Joshua that He would never leave him nor forsake him, what that unconditional? No, it was dependent on him keeping God's commands. Do we have proof of this? Yes, we do.
KJV Joshua 7:1 But the children of Israel committed a trespass in the accursed thing: for Achan, the son of Carmi, the son of Zabdi, the son of Zerah, of the tribe of Judah, took of the accursed thing: and the anger of the LORD was kindled against the children of Israel. {Achan: also called, Achar} {Zabdi: also called, Zimri}
2 And Joshua sent men from Jericho to Ai, which is beside Bethaven, on the east side of Bethel, and spake unto them, saying, Go up and view the country. And the men went up and viewed Ai.
3 And they returned to Joshua, and said unto him, Let not all the people go up; but let about two or three thousand men go up and smite Ai; and make not all the people to labour thither; for they are but few. {about...: Heb. about two thousand men, or, about three thousand men}
4 So there went up thither of the people about three thousand men: and they fled before the men of Ai.
5 And the men of Ai smote of them about thirty and six men: for they chased them from before the gate even unto Shebarim, and smote them in the going down: wherefore the hearts of the people melted, and became as water. {in...: or, in Morad}
6 And Joshua rent his clothes, and fell to the earth upon his face before the ark of the LORD until the eventide, he and the elders of Israel, and put dust upon their heads.
7 And Joshua said, Alas, O Lord GOD, wherefore hast thou at all brought this people over Jordan, to deliver us into the hand of the Amorites, to destroy us? would to God we had been content, and dwelt on the other side Jordan!
8 O Lord, what shall I say, when Israel turneth their backs before their enemies! {backs: Heb. necks}
9 For the Canaanites and all the inhabitants of the land shall hear of it, and shall environ us round, and cut off our name from the earth: and what wilt thou do unto thy great name?
10 And the LORD said unto Joshua, Get thee up; wherefore liest thou thus upon thy face? {liest: Heb. fallest}
11 Israel hath sinned, and they have also transgressed my covenant which I commanded them: for they have even taken of the accursed thing, and have also stolen, and dissembled also, and they have put it even among their own stuff.
12 Therefore the children of Israel could not stand before their enemies, but turned their backs before their enemies, because they were accursed:
neither will I be with you any more, except ye destroy the accursed from among you.
13 Up, sanctify the people, and say, Sanctify yourselves against to morrow: for thus saith the LORD God of Israel, There is an accursed thing in the midst of thee, O Israel: thou canst not stand before thine enemies, until ye take away the accursed thing from among you. (Jos 7:1-13 KJV)
God told Joshua, after saying He would never leave him nor forsake him that He wouldn't be with him any more unless he removed the accursed thing from among them. It's clear that the statement was not unconditional.
As I pointed out, Paul said these things were written for our example and our admonition that we might not do the same things. Israel sinned and God turned away from them, the Christian is no better, he has no special privileges, Scripture says that Jesus is the same yesterday, today, and tomorrow. Israel were God's people, they sinned and He turned away from them. Christians are God's people, when they sin there's no reason to expect special treatment that wasn't given to the Jews.