The literary context and argument must be kept in view when looking at this or any other passage. Paul has clearly affirmed his confidence of their salvation along with their calling to live as set apart believers. They had richly experienced God’s grace so that their testimony for the Lord had been clearly established, even through the bestowal of spiritual gifts in great abundance, lacking none. But in the context that follows, Paul points to their immaturity and carnality (1:10f). The need is for them to go forward into maturity and Christ-like change.
So, the apostle first affirms his confidence not only in their salvation as “those who had been sanctified in Christ,” but he wanted to affirm the availability of God’s sanctifying grace (vss. 8-9) by focusing them on three aspects of the grace of God: His work of confirming believers, His character as one who is faithful, and His means, through fellowship (koinonia) with His Son, Jesus Christ. The future tense of bebaioo “confirm, establish, strengthen” could well be a gnomic future, expressing not a particular event that will take place, but one that is true of any time. God is in the business of working to strengthen His people to conform them to the Lord Jesus. If it is a typical predictive future, then Paul is saying, they can know that God will work to fulfill His goals for believers, but, and this is very important, Christians are responsible to respond to the grace provision of God. Either way (gnomic or future), I don’t think this text is guaranteeing true saints will be confirmed blameless (perseverance) from the standpoint of sanctification. Positionally and judicially, yes, but not experientially (cf. 1 Cor 3:15; 11:28-32).
The rest of the book is a call to respond to the sanctifying and strengthening grace of God. If Paul were saying true saints will persevere, then what about those believers who died for failure to examine their lives and were careless with the Lord’s table as mentioned in 1 Corinthians 11:28f? Some were weak, some sick, and some were even asleep (dead) and this Paul defined as discipline from the Lord. The third group had not persevered, but they were believers.
Through fellowship with the Savior (sharing in the life of Christ in salvation and in our daily walk) in the inner man (1:9; Eph. 3:16f), we can count on His sanctifying grace to establish and change us spiritually, morally, and ethically. Spiritual change is not something we have to do on our own by our own spiritual push ups. But we do need spiritual discipline to appropriate His grace, and if we fail to do so, we can count on His fatherly discipline (1 Corinthians 1:28f; Hebrews 12:5f) to stop the downward direction even if it means the sin unto death.
I suppose many take 1 Corinthians 1:8-9 to refer to God’s preservation or the guarantee of the believer’s judicial standing as blameless in Christ as one who has been justified. This is certainly possible, but to me, this does not really fit the context as well because in the context, Paul is concerned with their Christ-like living which is certainly the opposite of the divisiveness going on at Corinth (1:10f). Being blameless can refer, as it is used in other passages by Paul, to a relative degree, in the sense of not of being sinless but of being without reproach as it is used of elders in 1 Timothy 3:10 and Titus 1:6. I think the same thing is true in Colossians 1:22.