(Matthew 6:24) No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon.
Here’s an excerpt from Ellicott’s Commentary on Matthew 6:24:
“(24) No man can serve two masters.—Literally, can be the slave of two masters. The clauses that follow describe two distinct results of the attempt to combine the two forms of service which are really incompatible. In most cases, there will be love for the one, and a real hatred for the other. The man who loves God cannot love the evil world, and, so far as it is evil, will learn to hate it. The man who loves the world will, even in the midst of lip-homage, hate the service of God in his inmost heart. But there are natures which seem hardly susceptible of such strong emotions as love or hatred. In that case there will be a like though not an identical, issue. The man’s will will drift in one direction or another. He will cleave to one with such affection as he is capable of, and will hold the other cheap. God or mammon, not both together, will be the ruling power with him.”
(Matthew 6:19-21) Lay not up for yourselves treasures upon earth, where moth and rust doth corrupt, and where thieves break through and steal: But lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through nor steal: For where your treasure is, there will your heart be also.
What is our heart serving? God or money? Both cannot be served, according to the Bible. God spiritually works in the believers, and makes it possible for them to enter the kingdom of Heaven through the strait gate and narrow way. Doing God’s will as a humble little child, not as a proud, carnally wise man who thinks he knows better than the omniscient God and His infinite wisdom. The Kingdom of Heaven is entered by being humble.
Jesus, as our ultimate example setter, was humble:
(Luke 22:41, 42) And he was withdrawn from them about a stone’s cast, and kneeled down, and prayed, Saying, Father, if thou be willing, remove this cup from me: nevertheless not my will, but thine, be done.
(Philippians 2:3-11) [Let] nothing [be done] through strife or vainglory; but in lowliness of mind let each esteem other better than themselves. Look not every man on his own things, but every man also on the things of others. Let this mind be in you, which was also in Christ Jesus: Who, being in the form of God, thought it not robbery to be equal with God: But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: And being found in fashion as a man, he humbled  himself, and became obedient unto death, even the death of the cross. Wherefore God also hath highly exalted him, and given him a name which is above every name: That at the name of Jesus every knee should bow, of [things] in heaven, and [things] in earth, and [things] under the earth; And [that] every tongue should confess that Jesus Christ [is] Lord, to the glory of God the Father.
(Matthew 18:1-4) At the same time came the disciples unto Jesus, saying, Who is the greatest  in the kingdom of heaven? And Jesus called a little child unto him, and set him in the midst of them, And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven. Whosoever therefore shall humble  himself as this little child, the same is greatest  in the kingdom of heaven.
Humility is the path to the Kingdom of Heaven:
(Mark 9:33-35) And he came to Capernaum: and being in the house he asked them, What was it that ye disputed among yourselves by the way? But they held their peace: for by the way they had disputed among themselves, who [should be] the greatest . And he sat down, and called the twelve, and saith unto them, If any man desire to be first, [the same] shall be last of all, and servant of all.
To desire to be “first” (KJV) is to not be humble. The believer in Christ is to seek to glorify God, not himself.
(Matthew 6:5) And when thou prayest, thou shalt not be as the hypocrites [are]: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have their reward.
The Kingdom of Heaven is entered by denying self, taking up one’s cross daily, and following Christ in humility, not pride:
(Matthew 6:6) But thou, when thou prayest, enter into thy closet, and when thou hast shut thy door, pray to thy Father which is in secret; and thy Father which seeth in secret shall reward thee openly.
(Luke 9:23) And he said to [them] all, If any [man] will come after me, let him deny himself, and take up his cross daily, and follow me.
(Luke 6:46) And why call ye me, Lord, Lord, and do not the things which I say?
So, the Kingdom of Heaven is entered by hearing the Lord Jesus (in the Bible, God’s word and truth) and following Him. And we know that only those who are born of God will be able to hear and follow Christ in love (and thereby do God’s will unto eternal life):
(2 Timothy 3:1-5) This know also, that in the last days perilous times shall come. For men shall be lovers of their own selves, covetous, boasters, proud, blasphemers, disobedient to parents, unthankful, unholy, Without natural affection, trucebreakers, false accusers, incontinent, fierce, despisers of those that are good, Traitors, heady, highminded, lovers of pleasures more than lovers of God; Having a form of godliness, but denying the power thereof: from such turn away.
(Philippians 2:3-11) [Let] nothing [be done] through strife or vainglory; but in lowliness of mind let each esteem other better than themselves. Look not every man on his own things, but every man also on the things of others. Let this mind be in you, which was also in Christ Jesus: Who, being in the form of God, thought it not robbery to be equal with God: But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross. Wherefore God also hath highly exalted him, and given him a name which is above every name: That at the name of Jesus every knee should bow, of [things] in heaven, and [things] in earth, and [things] under the earth; And [that] every tongue should confess that Jesus Christ [is] Lord, to the glory of God the Father.
Love is the proof test of whether a man or woman is born of God or not. No, not the worldly kind of love that condones sin. Rather, the kind of love that is expressed by doing righteousness without compromising what the Bible says about sin and moral conduct. Public opinion may change over time, but God’s holy word does not change.
The following passage contains what Christ will literally say to those on His left hand (according to the King James translation):
(Matthew 25:41-45) Then shall he say also unto them on the left hand, Depart from me, ye cursed, into everlasting fire, prepared for the devil and his angels: For I was an hungred, and ye gave me no meat: I was thirsty, and ye gave me no drink: I was a stranger, and ye took me not in: naked, and ye clothed me not: sick, and in prison, and ye visited me not. Then shall they also answer him, saying, Lord, when saw we thee an hungred, or athirst, or a stranger, or naked, or sick, or in prison, and did not minister unto thee? Then shall he answer them, saying, Verily I say unto you, Inasmuch as ye did [it] not to one of the least of these, ye did [it] not to me.
In comparison, the false prophets (described as “ravening wolves” (KJV) in Matthew 7) will bring up prophesying and casting out devils as noteworthy things that they did, supposedly in Christ’s name. Their words will not align with the recorded words of Jesus, Who will bring up giving food and drink, clothing, taking in, and visiting in prison, the least (doing charitable acts of love towards the needy who have nothing to offer in return), as noteworthy things that were not done by the “goats” on His left hand. Whether those who will be on Christ’s left hand are called “goats” or “wolves”, all who are not in Christ, and not born of God, will be on Christ’s left hand on that fateful day.
Notice how the words of Christ are to be followed, and those who who would be rich and suppose that gain is godliness are to be avoided:
(Acts 20:33-35) I have coveted no man’s silver, or gold, or apparel. Yea, ye yourselves know, that these hands have ministered unto my necessities, and to them that were with me. I have shewed you all things, how that so labouring ye ought to support the weak, and to remember the words of the Lord Jesus, how he said, It is more blessed to give than to receive.
(1 Timothy 6:3-12) If any man teach otherwise, and consent not to wholesome words, [even] the words of our Lord Jesus Christ, and to the doctrine which is according to godliness; He is proud, knowing nothing, but doting about questions and strifes of words, whereof cometh envy, strife, railings, evil surmisings, Perverse disputings of men of corrupt minds, and destitute of the truth, supposing that gain is godliness: from such withdraw thyself. But godliness with contentment is great gain. For we brought nothing into [this] world, [and it is] certain we can carry nothing out. And having food and raiment let us be therewith content. But they that will be rich fall into temptation and a snare, and [into] many foolish and hurtful lusts, which drown men in destruction and perdition. For the love of money is the root of all evil: which while some coveted after, they have erred from the faith, and pierced themselves through with many sorrows. But thou, O man of God, flee these things; and follow after righteousness, godliness, faith, love, patience, meekness. Fight  the good fight of faith, lay hold on eternal life, whereunto thou art also called, and hast professed a good profession before many witnesses.
(Luke 13:23, 24) Then said one unto him, Lord, are there few that be saved? And he said unto them, Strive  to enter in at  the strait  gate: for many, I say unto you, will seek to enter in, and shall not be able.
G75 – strive (3x), fight (3x), labour fervently (1x).
G1223- by (241x), through (88x),
G4728; Outline of Biblical Usage – narrow, strait
Here’s an excerpt from Barnes’ Notes on the Bible regarding 1 Timothy 6:12:
“Whereunto thou art also called – That is, by the Spirit of God, and by the very nature of your profession. God does not “call” his people that they may become rich; he does not convert them in order that they may devote themselves to the business of gain. They are “called” to a higher and nobler work. Yet how many professing Christians there are who seem to live as if God had “called” them to the special business of making money, and who devote themselves to it with a zeal and assiduity that would do honor to such a calling, if this had been the grand object which God had in view in converting them!”
To “strive” to enter in through the Strait Gate and Narrow Way, is to flee coveting after money and the many foolish and hurtful lusts associated with it, and follow after righteousness. To follow Christ by doing what He has said, is to “fight the good fight of faith”.
Here’s an excerpt from Ellicott’s Commentary on 1 Timothy 6:12:
““The good fight,” more closely considered, is the contest and struggle which the Christian has to maintain against the world, the flesh, and the devil.”
The Kingdom of Heaven is entered through the Strait Gate and Narrow Way, which is traveled by hearing Jesus and following Him by doing what He has said (in the Bible) and thereby do the will of the Father unto eternal life. Christ spoke the words of the Father. To follow Him is to follow the Father Who sent Him. Only believers who are in Christ, and born of God, are able to enter in through the Strait Gate and Narrow Way.